Don't eat Back button in TextEdit

On Android, Key_Back is used to change to the previous state in-app,
as well as to exit the application. QTextControl does not react to the
back button, so it should not accept the event.

Task-number: QTBUG-30752
Change-Id: I010d36c93f14384b96fb0b2f0015c57a2a3d2497
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>