move HotwordDetector ctor work to initialize()

new package-private initialize() method is added to
AbstractHotwordDetector to perform work which was previously done in the
implementing class' ctor. This allows the creator,
VoiceInteractionService to control when the initialize work is done
after create.

This change is apart of a larger topic to allow multiple detectors to be
active in parallel. By separating work done in the ctor to another
method, we can freely create detectors without impacting the other
services it communicates with.

This is an internal cleanup, and the client app using the detector will
observe no behavior change.

Test: atest VoiceInteractionSystemApiTest
Bug: 193232191
Change-Id: Ib63cff09e6a0c22fb0f77a0e721637dbb5ae7ed0