Fix background sensor usage counter

being out of sync with total count, due to nesting. Only update
the counter if nesting is 1.

Test: runtest -x
frameworks/base/core/tests/coretests/src/com/android/internal/os/BatteryStatsSensorTest.java
Change-Id: Iabb153550a79afa9902569bbbbdb5815decdc613
Fixes: 34750473