Copy BluetoothHearingAid API into private for short-term only

* SettingsLib will use APIs getDeviceSideInternal() and
  getDeviceModeInternal() to improve hearing aids experience and change
  to formal call in next bluetooth release.

Bug: 224323976
Ignore-AOSP-First: AOSP will be updated later
Test: add private APIs only
Tag: #feature
Change-Id: I7ccb399cfe9b4e1fbf37d752dacbdceeb09f5b8d