A2DP v1.4 uses the rfa bits for adding 5.1 and 7.1 configurations.
Clear those bits properly when sending configuration, in case remote
device sets them.
Make easier to package A2DP codecs separately, by splitting each to a
separate SPA plugin. Adjust the code to not use a global variable for
the codec list.
The A2DP SPA interface API is in the bluez5 private headers, and not
exposed in installed SPA headers, as it's too close to the
implementation.
