Add phone_account_* fields to voicemail content table allowed columns.

This allows updating and querying of voicemail entires for their
respective phone account columns.

Update tests to reflect this change:
- Increase number of voicemail columns to 18
- Add phone_account_* fields to update test

Bug: 19236241
Change-Id: I16b3c9d2a74bc7e169de4dd108b39b94acc98c4f
3 files changed