Torch: remind user flashlight is still on

Post a notification which informs the user the flashlight is still
enabled. But only do this when the user turns the screen off *while* the
flashlight is still on, so they see it next time they use the device.
Tapping on the notification will disable the flashlight.

Change-Id: I3689ff6498b97b813ccc10dc7dca3527fc8455aa
Signed-off-by: Roman Birg <roman@cyngn.com>