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.
Alternatively, we can leave the `targetSdkVersion` at 17. I don't think it's necessary to increase it to use the lock screen features. It's simply a hint to the system on what compatibility mode to use.