Step1 of renaming READ_WRITE_OWN_VOICEMAIL to ADD_VOICEMAILS.

READ_WRITE_OWN_VOICEMAIL sounds a confusing name. Dianne recommened to
rename it to ADD_VOICEMAILS as this simply allows 3rd party apps to add
new voicemails to the system. The fact that we allow the app to acces
only its own voicemail is implicit and need not be highlighted in the
permission name. See bug: 5098551 for more details

This CL implements the 1st step of this change by adding the permission
ADD_VOICEMAILS. A follow up CL will remove READ_WRITE_OWN_VOICEMAIL once
content provider and contacts app have been modified to start using the
new ADD_VOICEMAILS permission instead.

Bug: 5098551
Change-Id: I515e7967bdb0e8498a60a32983f9122ce10dcc4a