Fix power manager display wake lock bugs. (DO NOT MERGE)

Under certain circumstances, the power manager might continue to
hold the display wakelock long after the display had been turned
off due to the mDisplayReady flag having an incorrect value.

1. An inverted conditional caused DisplayPowerState to incorrectly
signal the screen on ready state.

2. The DisplayPowerController failed to clear the block screen on
flag in the case where the screen was turned off before it became
unblocked from turning on.  This could happen when the display was
rapidly turned on-off-on-off.

Bug: 13248135
Change-Id: I8faa3034695c83c8cd35613d81acccf40d22128d