Add getSubscriptionId API

The old getSubId or getSubscriptionIds return multiple sub ids
per slot index, which is not possible in today's implementation
because the slot index here refers to the logical slot index,
which is also known as phone id, or the logical modem index. In
today's telephony, one logical phone can only have one
subscription at one time, so does the modem.

Bug: 239607619
Test: Manual
Merged-In: I67cb2bade2c9496a50cccd81fa8f6edc57ede967
Change-Id: I67cb2bade2c9496a50cccd81fa8f6edc57ede967