floss: ConnectAllEnabledProfiles now connects A2DP

Add a MediaActions enum that we can dispatch to the mainloop so that we
can send actions from one part of the stack to other parts. Use this to
allow the Bluetooth interface to call Connect/Disconnect on the Media
interface and support A2DP in ConnectAllEnabledProfiles and
DisconnectAllEnabledProfiles.

Bug: 196887265
Tag: #floss
Test: Connect to audio device from btclient
Change-Id: Ib6911767b32de0e2f4be944d415f4f99084d273b