Refactor screen blocking logic.

Because of the call to turn on the screen introduced in ag/495255
is now made earlier we may not have sent the wakeup broadcast by
the time the windows have all drawn. In such a case onScreenOn()
did not release the wakelock and the screen stayed off.

This change ensures that no matter the timing of onScreenOn()
the wakelock will be released but only for the onScreenOn()
called in response to the most recent screen on transition.

Fixes bug 16404078.

Change-Id: Ic1e3e61e16c1fc34d4ec657e0e53ac49a05ca6a4