PhoneAccount -> SubId for use in TelephonyManager#line1Number

Since we do not want to expose SubIds in the app layer, we need a
wrapper to extra the subId from a PhoneAccount and return the value of
TelephonyManager#line1Number. TelecomManager exposes the method, this is
the implementation.

Bug: 19087382
Change-Id: I4cb293e83baebfd135a01e2ffd0a86b22a3d8917