PowerManager: Add proximity sensor support.

Add new wakelock flag PROXIMITY_SCREEN_OFF_WAKE_LOCK.
If you create a wakelock with this flag, while the wakelock is acquired,
the screen will turn off automatically when the sensor detects an object close to the screen.
Removing the object will cause the screen to wake up again.

Added PowerManager.getSupportedWakeLockFlags(), which can be used to determine
if proximity screen off wakelocks are supported by the hardware.

Signed-off-by: Mike Lockwood <lockwood@android.com>