Don't remove auto-connect priority for A2DP connected devices

The original adjustOtherSinkPriorities() implementation has a bug
that could incorrectly reset the PRIORITY_AUTO_CONNECT for A2DP
connected device to PRIORITY_ON.
Note: the bug is specific to A2DP and does not apply to HFP.

Also, added a corresponding unit test: testSetPriorityMultipleDevices

Bug: 73902689
Test: Manual: re-connect multiple A2DP/HFP devices
      Unit test:
  runtest bluetooth \
    --test-class com.android.bluetooth.btservice.PhonePolicyTest \
    --test-method testSetPriorityMultipleDevices
Change-Id: Ieb28afef95fb83259cbff74632ea7ea7d38445dd