Clean up PhoneAccountRegistrar.

Cleanup lots of duplicate code and consolidate all phone account
retrievals into a single method: getPhoneAccounts(...).

Consolidate checks for capabilities, package-name, uri, and
isVisibleForUser(...). This will be very helpful when
implementing enable/disable for phone accounts.

Bug: 20303449
Change-Id: I24d63cd313876f5d9390f2eb31c0ce79d7cb0ab2