Add Foreground Service Timer

Create a new timer to keep track of time spent in the Foreground Service
state. Expose said timer so Settings can use foreground service time in
its battery usage calculations.

Test: bit FrameworksCoreTests:com.android.internal.os.BatteryStatsNoteTest
Bug: 38313557
Change-Id: Ib30e9354665343262c6796c630b180a444bed88c