Remove ability to enable/disable phoneaccounts (3/6)

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 to getCallCapablePhoneAccounts

Bug: 17510811
Change-Id: I5e8a59fe2aaac2563fe4bf97138975e30b45aa3c