Release encoder state when cleaning up the hearing aids instance

When the Bluetooth state changes from ON to BLE_ON, the hearing aid
instance was cleaned up, but not encoder state. Since the Bluetooth
process is kept at BLE_ON but not exited, the new instance of the
hearing aids would see the encoder as initialized without starting a new
Bluetooth audio session, and caused the audio HAL to be unable to talk
to the stack. We now reset the encoder state when cleaning up the
instance, so it will start a session next time during the first
connection of a new hearing aid instance.

Bug: 127610666
Test: ON / OFF BT with BLE_ON and switch active device manually
Change-Id: I426fed4ea22c0b858bee273727fca6e2e7481e84