fix a dead-lock in sensorservice

sensorservice would deadlock if for some reason
a sensor failed to enable.

simplifed the code a bit, and made it behave a little
closer to mr1.1 -- I couldn't convince myself that
some changes in how locks were used were correct.

Bug: 9794362
Change-Id: I6110f5dbb67e543f1c71d127de2299232badb36a

Conflicts:

	services/sensorservice/SensorService.cpp
Add correct XHDPI heap size
Conflicts:

	build/phone-xhdpi-1024-dalvik-heap.mk