LE Audio: close GATT server properly

Currently we are leaking GMCS instance every time BT is restarted.

Bug: 273361967
Test: manual, restart bluetooth, observe output of:
adb shell dumpsys bluetooth_manager | grep -A 10 "Server\:"
(cherry picked from https://android-review.googlesource.com/q/commit:db6561f3d907c74055fe74f8e8a5b979da5cd1aa)
Merged-In: I3fabbe267a380a1eae482bb54be5551db9f0029e
Change-Id: I3fabbe267a380a1eae482bb54be5551db9f0029e