Handle volume changes for multiple devices

Since synchronizing volume of multiple HFP connected devices is rather
cumbersome as a UX, we will make the car control the volume of the call
and the phone's volume will remain unchanged. The only scenario that
this breaks is when the user is trying to change the phone volume while
being in a call and connected over Bluetooth. This scenario should
anyways not happen for dirving safety

Bug: b/34518638
Test: Volume change w/ multiple AG connected to HF
Change-Id: Ic4345a71bfb677f94cf6543b1b2c64fb99676ed3