stats_event_list changes for statsd mainline api

Cherry pick to aosp.
stats_event_list changes for creating a generic api for statsd mainline
logging.

Test: builds
Test: existing logs continue to flow
Change-Id: I4163edc80ffcca61db7ab2a1e14ae8c14aab2347
Merged-In: I4163edc80ffcca61db7ab2a1e14ae8c14aab2347
(cherry picked from commit 4c2f079d)