Enable HotwordDetectionService to access audioflinger.

HotwordDetectionService is an isolated service which ordinarily cannot
access audioflinger due to selinux restrictions on isolated processes.
To bypass those restrictions, the audioflinger binder is passed from
the system server to the HotwordDetectionService. This follows the
existing pattern of providing access to the ContentCapture service.

In T we *may* instead solve this at the selinux layer. For now, this
simpler approach is used.

Bug: 190011174
Test: manual - sample app can read audio (with a few other wip changes)
Change-Id: I86ac0820fd72676db4740658ffd096df3d20d30e