power_supply: Hold a wake_lock while power supply change notifications are pending

When connecting usb or the charger the device would often go back to sleep
before the charge led and screen turned on.

Change-Id: I01def6d86ddece0d4e31d2a91d176ed0975b6b9d
Signed-off-by: Arve Hjønnevåg <arve@android.com>