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.
This string is not really used because we use the 'summary' to display the current value of the setting (which is good!).