How to Set Up Screen Lock with a Password Programmatically

Programmatically changing ScreenLock mode from Slide to Pattern Android

You can use Device Administration API. And DeviceAdminSample is a good start.

There are following types you can set using setPasswordQuality()

        DevicePolicyManager.PASSWORD_QUALITY_UNSPECIFIED,
        DevicePolicyManager.PASSWORD_QUALITY_SOMETHING,
        DevicePolicyManager.PASSWORD_QUALITY_NUMERIC,
        DevicePolicyManager.PASSWORD_QUALITY_ALPHABETIC,
        DevicePolicyManager.PASSWORD_QUALITY_ALPHANUMERIC,
        DevicePolicyManager.PASSWORD_QUALITY_COMPLEX

There are several answers on SO

How can i set up screen lock with a password programmatically?
How can i lock the android device with a password programmatically

How to Prompt User to Enter Pin in Android Lock Screen?

You can use the below code to open password/pin/pattern screen and validate user device credentials:

if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.LOLLIPOP) {
    KeyguardManager km = (KeyguardManager) getSystemService(KEYGUARD_SERVICE);

    if (km.isKeyguardSecure()) {
        Intent authIntent = km.createConfirmDeviceCredentialIntent(getString(R.string.dialog_title_auth), getString(R.string.dialog_msg_auth));
        startActivityForResult(authIntent, INTENT_AUTHENTICATE);
    }
}

and also implement onActivityResult's method to get the results in your case is successful or not.

// call back when password is correct  
@Override  
protected void onActivityResult(int requestCode, int resultCode, Intent data) {  
    if (requestCode == INTENT_AUTHENTICATE) {  
        if (resultCode == RESULT_OK) {  
            //do something you want when pass the security  
        }  
    }  
}

You can check ref URL from here

Use screen lock in my app

Actually, there is an API to exactly that using the KeyguardManager.

First get a the Keyguard SystemService:

KeyguardManager km = (KeyguardManager)getSystemService(KEYGUARD_SERVICE);

And then request an authentication intent using:

Intent i = km.createConfirmDeviceCredentialIntent(title,description);

start this intent using startActivityForResult(Intent, int) and check for RESULT_OK if the user successfully completes the challenge.

This is for API level 21.
Previous versions might work with KeyguardLock.

How to programmatically set a lock or pin for an app

Logic

You have to make and start a service when you want to block apps,
And In Service you have to check packagenames of the apps, so that you can decide which app to run and which to show a pin/password activity

Now Code Example:

To Start a service, code like this,

startService(new Intent(this, SaveMyAppsService.class));

Now, Inside your service, check packages like this,

public class SaveMyAppsService extends android.app.Service 
{

    String CURRENT_PACKAGE_NAME = {your this app packagename};
    String lastAppPN = "";
    boolean noDelay = false;
    public static SaveMyAppsService instance;

    @Override
    public IBinder onBind(Intent intent) {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        // TODO Auto-generated method stub

        scheduleMethod();
        CURRENT_PACKAGE_NAME = getApplicationContext().getPackageName();
        Log.e("Current PN", "" + CURRENT_PACKAGE_NAME);

        instance = this;

        return START_STICKY;
    }

    private void scheduleMethod() {
        // TODO Auto-generated method stub

        ScheduledExecutorService scheduler = Executors
                .newSingleThreadScheduledExecutor();
        scheduler.scheduleAtFixedRate(new Runnable() {

            @Override
            public void run() {
                // TODO Auto-generated method stub

                // This method will check for the Running apps after every 100ms
                if(30 minutes spent){
                     stop();
                }else{
                   checkRunningApps();
               }
            }
        }, 0, 100, TimeUnit.MILLISECONDS);
    }

    public void checkRunningApps() {
        ActivityManager mActivityManager = (ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);
        List<ActivityManager.RunningTaskInfo> RunningTask = mActivityManager.getRunningTasks(1);
        ActivityManager.RunningTaskInfo ar = RunningTask.get(0);
        String activityOnTop = ar.topActivity.getPackageName();
        Log.e("activity on TOp", "" + activityOnTop);

        // Provide the packagename(s) of apps here, you want to show password activity
    if (activityOnTop.contains("whatsapp")  // you can make this check even better
            || activityOnTop.contains(CURRENT_PACKAGE_NAME)) {
            // Show Password Activity                
        } else {
            // DO nothing
        }
     }

    public static void stop() {
        if (instance != null) {
        instance.stopSelf();
        }
    }
}

Edit: (Get Top Package Name for Lollipop)

A very good answer is here.

android, Open lock screen settings programmatically

Possible duplicate of this SO Post

Have you tried doing this to navigate the user to a specific Settings Page -

Intent intent = new Intent(DevicePolicyManager.ACTION_SET_NEW_PASSWORD);
startActivity(intent);

For opening the Security Settings use this -

Intent intent = new Intent(Settings.ACTION_SECURITY_SETTINGS); 
startActivity(intent);

Moreover you can find here the constants for all the possible Actions that you can use in your app to redirect the user to a particular page on the Setting screen.

Let me know if this is not what you were looking for.