Fix dangling pointer issues post Bluetooth shutdown cleanup for delayed task in main thread.

Removed the Get() since it isn't used anymore.
Minor fix: Fixed HearingAid::Initialize() by returning if instance already present.

Bug: 260332122
Test: mma -j $(nproc)
Change-Id: I0411becd603ccfac7e77ef45c33f2560b3b02ac8
(cherry picked from commit ba0d0fc5)
Merged-In: I0411becd603ccfac7e77ef45c33f2560b3b02ac8