Fix volume controls show media instead of call volume during a call.

- On some apps like Signal, When the app is in the foreground and there
is a active phone call, volume controls show media volume control
instead of call volume control.
- Investigation showed that with apps having the right behaviour
"MediaSession.dispatchVolumeKeyEvent" is called, while with apps having
the wrong behaviour
"MediaSessionService.dispatchVolumeKeyEventToSessionAsSystemService" is
called.
- Following this guided to the cause of the steps to reproduce it in
other apps by calling "Activty.setMediaController".
- Issue was reproducible on the Sample MediaRouter App.
- Fixed it through checking not being in a call before forwarding the
volume event to the session.

Bug: 240705522
Test: Manual using Signal App / Youtube and Sample MediaRouter App.
Change-Id: Iabac3c80631fd429daba23454488fceedbcc584d