btif_av: access peers_ only from BtifAvSource/Sink

This CL adds mutexes to protect peers_ in BtifAvSource/Sink. It also
moves the invocation of btif_debug_av_peer_dump inside
BtifAvSource/Sink classes.
The Peers() methods are removed as they allowed nullpointer dereference in
some scenarios.
As peers_ are protected by mutex there is no need to handle
src_do_suspend_in_main_thread in main thread.

Bug: 300211201
Flag: Exempt, mechanical change only
Test: mmm packages/modules/Bluetooth

Change-Id: I509a41bff0c6200e11dc7c69705f942590e290a8