I was successful to lock/unlock my screen using DevicePolicyManager and KeyguardManager in Android L. It worked well when I lock/unlock screen using swipe mode (no security). However, I cannot lock/unlock it when I lock/unlock screen by using Pattern and Password mode (higher security). Is it possible to lock/unlock screen with high security using DevicePolicyManager and KeyguardManager. ? This is what I did
protected static final int REQUEST_ENABLE = 0;
DevicePolicyManager devicePolicyManager;
ComponentName adminComponent;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Button button = (Button) findViewById(R.id.btn);
    button.setOnClickListener(btnListener);
}
//LOCK
Button.OnClickListener btnListener = new Button.OnClickListener() {
    public void onClick(View v) {
        adminComponent = new ComponentName(MainActivity.this, Darclass.class);
        devicePolicyManager = (DevicePolicyManager) getSystemService(Context.DEVICE_POLICY_SERVICE);
        if (!devicePolicyManager.isAdminActive(adminComponent)) {
            Intent intent = new Intent(DevicePolicyManager.ACTION_ADD_DEVICE_ADMIN);
            intent.putExtra(DevicePolicyManager.EXTRA_DEVICE_ADMIN, adminComponent);
            startActivityForResult(intent, REQUEST_ENABLE);
        } else {
            devicePolicyManager.lockNow();
        }
    }
}; 
//UNLOCK
 private KeyguardManager keyguardManager;
 KeyguardManager.KeyguardLock kl;
 keyguardManager = (KeyguardManager) getSystemService(KEYGUARD_SERVICE);
 kl = keyguardManager.newKeyguardLock("MyKeyguardLock");
 kl.disableKeyguard();
Note that, I am using it in a service.
 
    