Apply Assistant AudioPolicy to HotwordDetectionService

The HotwordDetectionService is part of the Assistant package but runs
with a separate (isolated) uid.
This change follows the pattern of how the Assistant and IME UIDs are
set.

Fix: 194368677
Test: manual - hotword works when another app is using the mic and in
 Auto projection mode
Test: atest HotwordDetectionServiceBasicTest
Change-Id: I76a7930431db226c7e502f8993dfeaa4bc133bea