Android, Fix generic access to QAccelerometer

Generic access to QAccelerometer through QSensor was broken by my
previous change: 971fc95eca1d9db93aa4bdb75793dfe83e4a97db
When using a QSensor* to access the accelerometer, the qobject_cast
would fail, and return 0. I fix this by defaulting the accelerationMode
to Combined when a QSensor* is used.

Change-Id: If8389808a416807b72709a13ca315aa23bbf94a0
Reviewed-by: Bernd Weimer <bweimer@blackberry.com>
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>
Reviewed-by: Lorn Potter <lorn.potter@jollamobile.com>