Prevent Android from automatically locking display (#6876)

As mentioned in #5759

build/android/src/main/java/net.minetest.minetest/MtNativeActivity.java

@@ -10,6 +10,7 @@
 	@Override
 	public void onCreate(Bundle savedInstanceState) {
 		super.onCreate(savedInstanceState);
+		getWindow().addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
 		m_MessagReturnCode = -1;
 		m_MessageReturnValue = "";