Android: Remove AttachedJNIEnv class.

The JNI environment should be managed by QJNIEnvironmentPrivate
directly or through QJNIObjectPrivate. There is also a clear
difference between calls coming from or going into Java code.
Calls coming from Java already comes with the 'right' environment and
in most cases no extra considerations or set-up is needed.

Change-Id: I92d935ddfb70332041869185d5a92438930ff9b9
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>