Fix locked hardware buttons on Android

Fixes #2122
Fixes #1454

Addendum (est31)
According from its docs in android_native_app_glue.h (from the NDK), the
onInputEvent should "Return 1 if you have handled the event, 0 for any
default dispatching". Before, we always returned 1, meaning we blocked
all hardware keys to be given to the OS.
This broke the volume keys and has caused #2122 and #1454.

Although it bases on lots of guesswork, it can probably safely be said that
CGUIEnvironment::postEventFromUser returns true if the event was handled,
and false if not. Therefore, set the status variable depending on what
postEventFromUser returned.

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

@@ -19,6 +19,10 @@ public class MtNativeActivity extends NativeActivity {
 	public void onDestroy() {
 		super.onDestroy();
 	}
+	
+	@Override
+	public void onBackPressed() {
+	}
 
 
 	public void copyAssets() {