This is a statusbar that you see on top of your screen with icons of battry,clock ... .

let frame = require("ui/frame");

Hide:

frame.topmost().android.activity.getWindow().
getDecorView().setSystemUiVisibility(android.view.View.SYSTEM_UI_FLAG_FULLSCREEN);

Show:

frame.topmost().android.activity.getWindow().
getDecorView().setSystemUiVisibility(android.view.View.SYSTEM_UI_FLAG_VISIBLE );

open APP_Resources/values/styles.xml and add the

<item name="android:windowTranslucentStatus">true</item>

in the

<style name="AppThemeBase" parent="Theme.AppCompat.Light.NoActionBar"> </style> 

section.