LeAudio: Fix audio track metadata updates

Improve tracking of the currently active audio tracks, by allowing
for an empty audio track list updates. Without this change, the end
of life of particular track could not be precisely determined and
sometimes we end up with an obsolete audio context coming from an
obsolete audio track.

Bug: 388499616
Test: atest bluetooth-test-audio-hal-le-audio-software
Flag: Exempt: day to day fix
Change-Id: Ia8b098889d38cf41696b5793d6b1d5cb47b2d382