audio: Retain IBinder for instances with MinSchedulerPolicy

The binder passed to AIBinder_setMinSchedulerPolicy must also be
returned to the client, otherwise setting the policy for it does
not make any sense. However, server side interface instance
classes only hold a weak binder reference. It's the caller of the
'asBinder' method who must retain a strong reference. This
reference must be retained past exiting from the method which
returns the instance to the client.

To solve this issue, add storing of binders along with server
object references. These binders get released after the client
calls a 'close'/'destroy'-type method to release instance
resources.

Bug: 205884982
Test: run `atest VtsHalAudioCoreTargetTest` and effect VTS,
      and grep logcat for
     'destroyed after setMinSchedulerPolicy before being parceled'
Change-Id: I8b905b85cb8263c85edae8839a126ffe4e4d1e69