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>
WinRT: Check whether default sensors were obtained successfully

In case of an unsuccessful call of the GetDefault function the returned
HRESULT does not have the FAILED status but the returned sensor is 0.

Task-number: QTBUG-39698
Change-Id: I42a8327c1d7687ab7a8ca71be5a2234b2c4e0b38
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Added tizen sensors plugin

Change-Id: Ib455e5c53be47ae4910f56b5b714a4a44e6a8fb3
Reviewed-by: Witold Wysota <wysota@qtcentre.org>
Reviewed-by: Tomasz Olszak <olszak.tomasz@gmail.com>
Added autodetection of Tizen sensors.

Tizen sensors are available for mobile and common profiles.

Change-Id: Icc3c3357e932dac3808aee891e467fcc9dad2181
Reviewed-by: Philippe Coval <rzr@gna.org>
Reviewed-by: Tomasz Olszak <olszak.tomasz@gmail.com>