WifiLockManager: Use WorkSource.isEmpty instead of WorkSource.size.

Required to support WorkSources that contain WorkChains (chained
attribution). This change also adds smoke tests to make sure that this
works end-to-end from the WifiLockManager's perspective.

Test: WifiLockManagerTest
Bug: 62390666
Change-Id: I4abb22c62bf9d21e4a661dcc4af2fb1b4aa0efd1
2 files changed