Clean up SubMgr#getOpportunisticSubscriptions.

Adding more descriptive comments for this API.
Remove param slotIndex.

Bug: 117832560
Bug: 117830188
Test: build
Change-Id: I906cfb8ccb1851bde6be09c273d47092337ed1c1
Merged-In: I906cfb8ccb1851bde6be09c273d47092337ed1c1
3 files changed