audioflinger: filter out reserved keys from setParameters()

Only allow setParameters() for reserved keys when received from
audioserver UID. For instance, keys used to control routing or audio
stream configuration are reserved for use by audio policy manager.

Also use multiuser_get_app_id() instead of duplicated code to
extract application ID from UID.

Bug: 77869640
Test: manual audio smoke tests.
Change-Id: I88852e8fddf7f705e4a084fc02d9ced1f4b0de92