Add new fields - |upstream_events| and |duration_millis| in stats.proto

Revise annotations in previous fields and deprecate |upstream_type|.

Ignore-AOSP-First: Merge conflict if submits in AOSP first.
Bug: 153942334
Test: statsd_testdrive 303
Change-Id: Id4d594f9238e18266dce5bf2ba238c1f86f655db
(cherry picked from commit 71c8495c)
Merged-In: Id4d594f9238e18266dce5bf2ba238c1f86f655db