change KeyphraseEnrollmentInfo to store services

Current hotword enrollment is done through a transparent activity which
when triggered causes a breif interuption on the UI. Because there is no
actual UI component to this activity, it is being transitioned from an
activity to a service.

applications supporting MANAGE_VOICE_KEYPHRASES are moved from ativity
to service based to allow for enrollment which does not effect the UI

Bug: 139954183
Test: confirmed enrollment and unenrollment intents can start the
registered enrollment service.

Change-Id: I1e2d80e08efaf47a5c1c8760e2979143df436a93