Disable MediaMetrics for the HotwordDetectionService.

HotwordDetectionService is an isolated service which ordinarily cannot
access the audio system due to selinux restrictions on isolated
processes. To bypass those restrictions, the AudioFlinger binder is
being passed from the system server to the HotwordDetectionService.
(I86ac0820fd72676db4740658ffd096df3d20d30e)
To allow reading audio, accesses to the MediaMetrics service must also
be handled. Instead of similarly passing the MediaMetrics binder, we
take the simpler approach of just disabling the service there for now.

Bug: 190011174
Test: manual - sample app can read audio (with a few other wip changes)
Change-Id: I740b4f5acb88821197e3c4023a4032babd48ec5a