Lollipop lock screen notifications

Add support for lock screen notifications on Lollipop.

Vectorize the icons used there (hooray Illustrator).
Bump SDK and support-v4 to 21.0 to get lock screen notification API support.

No changed for notifications on pre-Lollipop devices.
Hmm, I'm not sure this makes much sense given this code deals both with the public and the non-public notification, so the scope of that new method gets a bit unclear. it would make sense to put the sender list generation into a new method, though.