Wait for Notifications before sending audio data

For the Hearing Aids Profile, wait for the notification status from the
devices after sending the Start command. The reason is that the devices
may be in low power mode and the phone should not start streaming audio
data to these devices until it is fully awake.

Test: Manual testing with Hearing Aids devices
Bug: 117778955
Bug: 120921220
Change-Id: I28b79e7e284d5e75864c0b12315fbb8bd47acf74