audiod: Handle crashes of para_server more robustly.

audiod: Handle crashes of para_server more robustly.

In case para_server dies unexpectedly, para_audiod may still have a partial status
item in the buffer tree node of the client task which can lead to a busy loop.

Fix this by flushing the input queue and invalidate the current audio format to prevent
recreating a buffer tree before the status task resumes.

There is already flush_input_queue() in buffer_tree.c but that can't be called by audiod.c
as it is a static function. Make it public and rename it to btr_drain() which is shorter and
more to the point and has the usual btr_ prefix.