hal: Send A2DP backend config for each routed stream

There exists a corner case where A2DP backend config is not
sent after BE is closed and reopened leading to slimbus
running at default rate. This happens because A2DP snd_device
is already active, hence backend config for A2DP is not sent.
However, audio route is disabled and enabled i.e. BE is closed
and reopened. Thus, slimbus is configured at 8 KHz which is a
problem for A2DP.
Fix this by sending backend config for each stream that gets
routed to A2DP and A2DP snd_device is enabled.

Change-Id: I3edfcaaac9d0bc81f7be4090f8057f3947b1e5d0