SystemUI: add flashlight notification reminder

CM13 port of change-id I3689ff6498b97b813ccc10dc7dca3527fc8455aa.

We now only post a notification when the user initiated the flashlight
via the QS tiles (or "V" gesture) since there is a common API for flashlight apps to hook
into (and possibly place their own notification, which we don't want to
ever double up on).

Ref: CYNGNOS-1670

Change-Id: Id532e8449932fefb216c10798aab6241f6e491ea
Signed-off-by: Roman Birg <roman@cyngn.com>