PowerManagerService: Clear Binder calling identity around calls to Sensor Manager.

This fixes a problem that resulted in bogus battery statistics for the prox and light sensors
due to the sensors being enabled in an app's context and disabled in the system context.

Fixes problems reported in bug b/2211960

Change-Id: Ib0d3d9d06f043b386d3b8d494b9bad5bd40bb6e1
Signed-off-by: Mike Lockwood <lockwood@android.com>