bluez5: add aptx-LL codec

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.