Mark 'session' field in audio HAL AIDL as unused.

The 'session' field is only used by the framework (if at all),
so it being moved to the HAL AIDL to the system-specific part.

Also update the comment on AudioPort.

Bug: 205884982
Test: m
Merged-In: Ia214c343bbf08c7944ea5ebf6140a92821941b46
Change-Id: Ia214c343bbf08c7944ea5ebf6140a92821941b46
(cherry picked from commit 8abc0314a7e6c6b6569d5e2a21e17c3040101d32)
2 files changed