Add REMOVE_VOICEMAIL permission to system

Add a permission that will allow any app that has this permission
to remove any voicemails in the user's call log, even those that
don't belong to it.

This will be needed for the unbundled dialer to be able to manage
the user's voicemails.

Bug: 6948882
Change-Id: Idca23ee83e541ba8aef24d6cb7fe09d5adac1e10