Make EXTRA_IS_REFRESH for voicemail notification public

This extra is added in ag/2333070 to let the dialer know if a
notification should be silent since the user has alread been notified

Change-Id: Ib2b4ab4cfa1c4baaef4eceb6fa1dab6a8fcfb84d
Fixes: 62202833
Test: manual - leave voicemail, toggle airplane mode.