Add API method to extract subscription ID from phone account (1/3)

Since Telephony uses subscription ids and Telecom uses Phone Accounts,
there should be an easy way to interface between the two. Since the
value of the subscription id and "id" of the phone account should be the
same in the case of a SIM phone account, we just need to check that the
phone account in question is a SIM phone account and convert the string
to a long.
This lays in groundwork for the following bugs:

Bug: 17925501
Bug: 17917937
Change-Id: I926e969b365d9e9878436302117e8786ec5513b8