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.
Please don't build user-visible text by concatenating strings in code. Make this a format string in `strings.xml`.

While you're at it, it would be nice if you could change `notification_new_messages_title` to be a plural resource (we don't want to see "1 new messages").