Floss: Clean up message loop threads when SIGTERM

When terminating btadapterd, we need to gracefully close all of the
message loop threads in libbluetooth otherwise it might crash.

This CL implements the cleanup function for BluetoothMedia as it uses
some threads that doesn't close in btif cleanup.

Bug: 335146967, 304997914, 353900236
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: run below test mannually while a2dp and hogp is in used
      1. rmmod btusb && modprobe btusb while
      2. hcitool cmd 3f 4e (to trigger hardware error)
Flag: EXEMPT, Floss-only changes
Change-Id: I50ede04357bf8a24efd3d29e96c481961aec42d7