hal: Listen for battery charge/discharge events

Register audio HAL as a client of health HAL
to get battery state changes. Audio HAL will
use this hint to make decisions locally.
Currently used to send events to sthal for
VA transition from low power WDSP to high
performance ADSP solution.

Change-Id: Iaad8530059c36b5e72f382c4df64c8561307c414