Track background sensor usage

Keep an additional counter for background access
of sensors. Increment the count when sensor is
accessed while process is in the background state.

Bug: 33553631
Test: runtest -x frameworks/base/core/tests/coretests/src/com/android/internal/os/BatteryStatsSensorTest.java
Change-Id: I87d7dbdaf4c34aa9dd9c7e13134d8f5f759f4e2b