app_stream_echo: misc bug fixes

Fixed the following bugs:

* calls to stream_echo_write had the last two parameters swapped
* ast_read should have been ast_read_stream
* added a null check on the frame's subclass format

This also resets the update_sent flag upon receiving SRRCHANGE control frame.
This will then force a video update.

ASTERISK-26997

Change-Id: I6ad7c8253559b800800433c52339e7f5aa583566