Fixed multihal handling of nonexistent sensor handles.

This prevents crashes and undefined behavior when someone
calls sensor HAL methods with nonexistent handles.
It also adds checks and warnings if bad sensor HALs provide
garbage sensor handles in their sensor events.

Tested by making sure pedometer and tilt still work.

Bug: 13513595
Change-Id: Ie1489d3f2972bec82ee1c73189e532f6658d1eca
1 file changed