Add Sub-ID-from-phone-account converter method.

There are several places in the code that currently attempt to parse the
sub ID directly from the phone account which is undesireable.  Before we
can change from using the subID to using an ICC ID with the phone
accounts, we need to create a conversion method so that no other
components are parsing the sub ID out of the phone account directly.

Change-Id: I74547fe0807da2fe13f3ff4422cd1d0a7a20d04c