Remove ability to enable/disable phoneaccounts (1/6)

Enabling/disabling of phone accounts was only used for SIP accounts and
is no longer necessary for the purpose it was put in.
- Remove all references to enabling/disabling phone accounts
+ Rename getEnabledPhoneAccounts -> getCallCapablePhoneAccounts

Bug: 17510811

Change-Id: Id805daae3cc698c75f5e747b6a6107ab0bd3afa0