Support the low-latency variant of the aptx codec.
The magic mostly seems to be on the device side, since the stream is the
same as standard aptx, but latency is smaller even if stream/packet
sizes are the same.
Sound output latency is noticeably less than with the standard aptx.
Tested on Sennheiser HD 250 / Avantree Aria Pro.
The codec in principle also supports bidirectional duplex streams,
but that is not implemented here.
libfreeaptx is a fork of libopenaptx prior to the dubious licensing
situation was introduced to the library.
As it's fully API compatible, let's use that instead for those who
want to use aptX support.
The library source is available at https://github.com/iamthehorker/libfreeaptx
Mark some structures, arrays static/const at various places.
In some cases this prevents unnecessary initialization
when a function is entered.
All in all, the text segments across all shared
libraries are reduced by about 2 KiB. However,
the total size increases by about 2 KiB as well.
Let codec decides when rtp packet need to be sent (terminated by MTU size in most case).
LDAC encoding loop can now be terminated by reading if frame_num is written, no 'frame_count' updating is needed.
RTP payload fragmentation can now be implemented more easily based on this.
Initial Props value are parsed from device settings, further changes are triggered by 'set_param' on a2dp node.
Codec can then use props to tweak its transcoder.
