bass: Get subgroup number index from metadata

For local broadcast there is no Periodic Advertisement result. Previous
solution assumes that updated source was previously external broadcast.

Current solution just get the same information from metaData object
which also contains information about subgroup.

Tag: #feature
Bug: 290883060
Test: atest packages/modules/Bluetooth/android/app/tests/unit/src/com/android/bluetooth/bass_client
Change-Id: Ifb7989c5645358e117cbf6abfd52ca30a9b200aa