BatteryStats: Add WorkChain support for WiFi events.

The changes are straightforward, the only change outside of BatteryStats
is to use the new WorkSource.isEmpty API to account for WorkChains in a
given WorkSource.

Bug: 62390666
Test: BatteryStatsBackgroundStatsTest, BatteryStatsNoteTest, WifiLockManagerTest
Change-Id: I1dff43b6f2a09877e3af4442bfe8a8fd80b1ba74