Power Manager light sensor fixes:

Fix problem with ignored sensor events right after screen turns on.
Do not flash button backlights when device is booting.
Process the first light sensor change after waking the screen immediately rather than debouncing.
Cancel mAutoBrightnessTask when light sensor is disabled.

Change-Id: Iffc5e090b239432b7643812fa8ecb5f69da1679d
Signed-off-by: Mike Lockwood <lockwood@android.com>