Simplify codec switching code: determine what switch to perform
immediately in spa_bt_device_ensure_media_codec().
The previous code doing "fallback" switching to various codecs is not
useful, as A2DP generally disconnects on the first failure and all
remote endpoints disappear.
Add iteration over multiple endpoints, for reconfiguring both source and
sink directions at the same time. This is in preparation of supporting
BAP reconfiguration (for A2DP there's usually only one direction
connected at a time).