Android: don't crash if Gravity is not supported

The QtSensors documentation says that if Gravity mode is not available,
we should silently fall back to the default Combined mode.

In addition, add null pointer checks in case we try to ask for non-existent
sensors in the future.

Task-number: QTBUG-39335
Change-Id: I6c18622d081db4e3a7d07c744bb0b746e3f5c6ff
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>