Enhance AlwaysOnHotwordDetector.EventPayload with more fields.

This will eventually support a more flexible information handling:
passing HotwordDetectedResult and audio stream directly from the server.

Bug: 182788844
Bug: 168305377
CTS-Coverage-Bug: 183425641
Test: N/A committing data class, will test e2e
Change-Id: Ie97723c113bced3046a1d2a652a95f94399399be