Fix race condition resulting in unbalanced wakelock calls

Because we release the lock during threadLoop(), we might actually
make the queue empty more than once during a cycle, but only release
the wakelock once in the end.

This change makes sure we release once for every time we make the
queue empty, just like we acquire once for every time we make it non-
empty.

Test: No good way to repro. Will submit and wait for the power monitor
      results.
Bug: 228012293
Change-Id: I740b438dbddb6aace41263427bde95f902ed25a0