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.

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jack_iodelay shows 50-70 ms roundtrip, but on ALSA I get 40ms (with same microphone), so seems OK for Bluetooth.

XXX: This preferably should be tested with more devices. Tested with two.

XXX: I had to bump the requested buffer levels up by a bit from the a2dp-codecs.h "defaults". This might be solvable by changing the way the socket writes are done, but OTOH it's ~10ms increase in latency. Maybe can be done later if it turns out to be a problem.

See #972 (closed)