Tweak API docs/annotations for API council review

Tweak API docs/annotations after API council review:

1) Add @CurrentTimeMillisLong annotation.
2) Remove mention of "confidence" from the docs. The focus here should
   be on what the UI is expected to do, not why.
3) Remove leading "m" from properties docs. Java POJO properties, in
   proper JavaBeans specification style, are what comes after the "get"
   / "is" / "set" (which also works best with Kotlin).
4) Move @ElapsedRealtimeMillis position: it's a method annotation, not a
   type annotation currently.

Test: treehugger only
Bug: 255592891
Change-Id: Id988f30226b18f1762df164607ba68fb554040e7