Fixes vsync behavior in power mode transitions.

Calling EventThread::onScreenAcquired enables hardware vsync and
calling EventThread::onScreenReleased disables hardware vsync.

The power modes 'NORMAL' and 'DOZE' should have hardware vsync
enabled while power modes 'OFF' and 'DOZE_SUSPEND' should have
hardware vsync disabled.

This change correctly handles the transitions from 'OFF' to
'DOZE_SUSPEND' and from either 'OFF' or 'DOZE_SUSPEND' to
'NORMAL'. Previously these cases were handled incorrectly.

Note that redundant calls to onScreenAcquired or onScreenReleased
are safe.

Fixes bug: 38232221

Change-Id: I7b00f97a67b157366364b3d26fe94533da07c263