stats_event.h/c tests

Add unit tests for the native API to log atoms to statsd.

(This CL is being cherrypicked into AOSP in order to prevent automerger
issues with the rename from libstatssocket to libstatspush.)

Test: bit libstatssocket_test:*
Bug: 145231901
Change-Id: If427c17319787200260cbe3b71075ca556c9a82b
Merged-In: If427c17319787200260cbe3b71075ca556c9a82b