Keep screen on while scanning QR code.

briar-android/src/main/res/layout/fragment_keyagreement_qr.xml

@@ -3,7 +3,8 @@
 	xmlns:android="http://schemas.android.com/apk/res/android"
 	xmlns:tools="http://schemas.android.com/tools"
 	android:layout_width="match_parent"
-	android:layout_height="match_parent">
+	android:layout_height="match_parent"
+	android:keepScreenOn="true">
 
 	<org.briarproject.briar.android.keyagreement.CameraView
 		android:id="@+id/camera_view"