Add Feature Flag for Hearing Aid Profile

Using the Settings App-Developer Options-Feature Flag, allow the user to
enable or disable the Hearing Aid Profile.

Bug: 116317072
Bug: 116044083
Test: Manual testing using Settings App
Merged-In: I58a9d339941e235242c443c85b6f4194b5a296c9
(cherry picked from commit dea97b7e)

Change-Id: I7d0b6759e7acba3fb2a4d1afdba35d5a95387777