Added public notification for calls

+ Previously we didn't have a notification for ongoing/incoming calls
to show users when they're on the lock screen and have their
notification settings set to 'hide sensitive notification content'.
This change adds that notification, to disambiguate between
notifications for calls and other notifications such as those for
missed calls.
+ This notification matches the version shown when the device is
unlocked except that we choose to not show work call information as
that is related to contact info.

See http://screen/zYZseSS1d54

Bug: 13387034

Change-Id: I02290e585a9b46eb9207adf8258a67d29882518b