AudioDeviceBroker: fix deadlock with setA2dpSuspended

There is a potential deadlock between AudioDeviceBroker and
HFP HeadsetService when AudioManager.setA2dpSuspended (or
setLeAudioSuspended) is called.

To avoid it, process the suspend requests in the message handler
instead of synchronously.

Bug: 278668288
Fixes: 279558132
Test: repro steps in bug
Change-Id: I83e3a5e918224f63f39438e30221db06f1dc2ad0