Add persistent state for Hearing Aid Profile

Defines the flag used in code and the Settings.Global key names for
persisting state of the Bluetooth Hearing Aid Profile.

Test: manual
Bug: 69623109
Change-Id: I19e68be55a80f338bdc5d39d6c3c6d251f1cb514
Merged-In: I19e68be55a80f338bdc5d39d6c3c6d251f1cb514