Allowing enable/disable of phone accounts. (3/3)

Phone Accounts:
- Added Phone account enable/disable activity, accessible from the
"Make calls via" preference screen.
- Changed Telephony PhoneAccount registration so that it first updates
existing PhoneAccounts (ensuring enable/disable setting retained), and
then removes any that no longer exist.

SIP:
Removed the "primary profile" SIP setting in favour of being able to
enabled and disable the associated PhoneAccounts.
Added migration to automatically enable the previous "primary profile".

Bug: 17306514
Bug: 17408536
Change-Id: I696e3382730ea7bb40d680798bcab965517c438b
16 files changed