add support for multiple keyphrase detectors

This change allows for the active VoiceInteractionService to create any
number of unique DSP-based AlwaysOnHotwordDetectors in parallel. This
change also allows for a SoftwareHotwordDetector to be created alongside
the DSP-based detector(s).

When a implementation of VoiceInteractionService has a target SDK lower
than Android T SDK version, there is no behavior change, and the
application will only be able to use a single HotwordDetector at a given
time.

Test: atest VoiceInteractionSystemApiTest
Bug: 193232191
Change-Id: I4bc7211884d2d63dc7fcda059384f3a8233a41a8