AOHD.EventPayload.getHalEventTimestampMillis API

AlwaysOnHotwordDetector.EventPayload API for exposing the timestamp
which represents the time the SoundTrigger HAL emitted a
RecognitionEvent

Test: atest CtsVoiceInteractionHostTestCases
Bug: 265852186
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:51d3a40d4bee1fa4f8f44ec4e59380911ffa0684)
Merged-In: I0f8d37e19002910bbfbf3bfd77cdecbf7c588e81
Change-Id: I0f8d37e19002910bbfbf3bfd77cdecbf7c588e81