HFP: Fix AG indicator update for Multi-HFP

* Keep AG indicator subscription when at least one HFP device is connected
* Respect AT+BIA commands by turnning indicator subscription on and off

Bug: 77655992
Test: make, HeadsetPhoneStateTest, HeadsetStateMachineTest
Change-Id: Ifa4a1c2f730b286f4f8d35fdec92e37f05f02b67
(cherry picked from commit 53f33f4b)