Add callback for output stream.

The callback is targeted for events related to an output stream.
Currently, there is one callback event defined, which is codec
format changed event.

Bug: 133526565
Test: manual
Test: atest VtsHalAudioV6_0TargetTest
Change-Id: I73a4914c1ffc30e1c88b8fedd61a031e24a069f6