hotword: Add background audio signal power property

An integer property representing background audio signal power is added
to the HotwordDetectedResult class. The property can have value
in the range [0, 255].

This change is P1 since it's based on customer's feedback for API.

Bug: 273814976
Test: CTS
Change-Id: Ifaee710cd7dab7e1a70dc5277195af34ce8ce562
2 files changed