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.
The main reason for wanting another method is that it is already way too long.
The code in this `if` body is concerned with setting up the lock screen notification. That is a narrow enough scope for me.