Call Index: To Handle HFP index change after call Merge and disconnect

Bug: 318460024
Bug: 345380335
Test: Manual | Make two incomming calls, merge calls & disconnect first call
Test: atest BluetoothInCallServiceTest#conferenceLastCallIndexIsMaintained
Flag: com.android.bluetooth.flags.maintain_call_index_after_conference
Change-Id: Ibf7015d7a250d62641a9e1df417d17d75bad71b6