Merge branch '1247-flag-secure-warning' into 'master'

Add warning to FLAG_SECURE about app locking implications

Closes #1247

See merge request !922

briar-android/src/main/java/org/briarproject/briar/android/activity/BaseActivity.java

@@ -68,6 +68,11 @@ public abstract class BaseActivity extends AppCompatActivity
 	public void onCreate(@Nullable Bundle state) {
 		super.onCreate(state);
 
+		// WARNING: When removing this or making it possible to turn it off,
+		//          we need a solution for the app lock feature.
+		//          When the app is locked by a timeout and FLAG_SECURE is not
+		//          set, the app content becomes visible briefly before the
+		//          unlock screen is shown.
 		if (PREVENT_SCREENSHOTS) getWindow().addFlags(FLAG_SECURE);
 
 		AndroidComponent applicationComponent =