Bluetooth: audio: Fix bt_pacs_context_available

This fixes bt_pacs_context_available so that it gets available
contexts from capabilities that hold the value.
This removes redundant available_context that were held
(and not updated) in pacs.c.

Fixes: HAP/HA/STR/BV-01-C
Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>