Public StatsLog API for generic mainline logging.

This is a cherry pick to aosp.

Sets up a public api for logging atoms to statsd. The API excepts a
buffer which is already encoded in the proper format for the socket, as
well as the number of bytes of the buffer to write. It performs a JNI
call to perform the socket write. Autogenerated app code will be built
for each mainline module that needs to use this API to log.

Test: builds
Test: existing logs flow properly
Bug: 126134616
Change-Id: I8a9a91e638d730e3ff69cb9345692e49e0db3c96
Merged-In: I8a9a91e638d730e3ff69cb9345692e49e0db3c96