Fixing bug in donation settings

in cl/173731907 we added a setting for donating voicemails, but didn't
distinguish between when this feature is enabled/disabled by the user
or by configuration.  this cl fixes that by adding a method to check
if the feature is available (ie enabled by config).

Bug: 62423454
Test: manual and unit test
PiperOrigin-RevId: 174245260
Change-Id: I45a59de7f4a171d759e2fcf86db27a100a84de67
5 files changed