Fix the use of WakeLock.

WakeLock is reference counted, so we need to call release() once for
each acquire(). Otherwise when WakeLock is GC'ed it kills DocumentsUI
by Log.wtf(). Unit tests will then fail to run on eng build.

Test: Tests pass.
Change-Id: I98889e1d99c9b814abf0746de1cfbc59737f05d2