Allow voicemail notification dismissal to be configurable.

If a device/carrier allows it, a voicemail notification in the
notification bar should be dismissable. Note that this change does not
wipe the notification completely, i.e. it will come back if the user
restarts the phone.
Add configuration values relevant to Verizon for non-dismissable
notifications.

Bug: 7888110
Change-Id: I7af4a610fb2ba330633157abcc59f8940372fd85
6 files changed