Fix NPE in WakeLock#finalize for unit test

The stubbed method of "mock(PowerManager.WakeLock.class)" will
lose effect after clearInlineMocks. And then finalizer runs
the real method finalize() that cause NPE because the mocked
object didn't initialize fields.

Bug: 219640050
Test: atest WmTests

Change-Id: If8ee4fbc98c9e33d6ab6ba75a65ba177fa06f09c