Fix crash when toggling ambient collapse button in hearing device dialog

Root Cause: mAmbientController.start() in onStart() might execute before mAmbientController is
initialized in onCreate(). Race condition between background executor in onCreate() and onStart()

Solution: Move task in onStart() to onCreate() to make sure it will be executed in order.

Bug: 397635040
Test: manual test, connect one of hearing aids then open hearing device dialog
Flag: EXEMPT bugfix
Change-Id: Ic9e6f1805789fbf26437ae54b5c68dcec8e17d7a