Revert "[hotword] make service exemption configurable via sysprop"

Revert submission 23212396-hotword_exemption_sysprop

Reason for revert:  Potential culprit for b/282730720 - verifying through ABTD before revert submission. This is part of the standard investigation process, and does not mean your CL will be reverted.

Reverted changes: /q/submissionid:23212396-hotword_exemption_sysprop

Change-Id: I75b6f44f39db54234f47f7b63dfdc5d5e0cde1e2