Expose HWUI metrics via statsd

Add atom definition for HWUI stats. Implement a C++
statsd puller inside GraphicsStatsService service.
GraphicsStatsService has new private API, which
returns a serialized proto with HWUI stats grouped
by application package and version.

Test: Ran "adb shell cmd stats pull-source 10068"
Test: Ran "statsd_testdrive 10068" and it looks OK
Bug: 142665516
Change-Id: I400c0dbf9e25181d36f9018688b03d86839ac3de