avmodel: use synchronous callbacks for DecodingStarted/Stopped

This is an older issue that has resurfaced where mobile device rotation
at the beginning of a call cause a frame-copy to a stale buffer because
the DecodingStarted event is handled asynchronously.

Noticed on Windows but I believe any non-dbus build should have it.

So we make all the connections blocking and adjust some parameters.
This commit also removes the DecodingStarted handler in CallModel which
was causing the client's target video frame to be reallocated for each
account present.

Change-Id: I23ac4e0bd4b446e7a532f0d362f7ecd209d3c790
GitLab: #536