OutputTrack: Ensure weak pointer is held outside of lambda creation.

Ensure there are no races with respect to capture initialization,
construction, and locking.

Flag: EXEMPT bugfix
Test: Play notifications with Sony 1000XM4 (duplicating thread), toggle BT on/off 10 times.
Bug: 349514538
Change-Id: If63cb0306a8184648e3ba8034c2f081927ec016d