Allowing enable/disable of phone accounts. (1/3)

- added "hasCapabilities" method on PhoneAccount.
- added CAPABILITY_PLACE_EMERGENCY_CALLS.
- added isEnabled method builder constructor to initialize from an
existing PhoneAccount.

Bug: 17306514
Bug: 17408536

Change-Id: I57de508b4adcf207f3b29cab449bfc634db80153