Update GraphicsStats proto definition

Follow ProtoBestPractices and comments suggested by cl/295218381.
Changing proto definition, should be fine, because these are
fields added after the last Android release.
Move PipelineType enum inside GraphicsStats and add a default
UNKNOWN state.
Rename start_millis and end_millis in GraphicsStats atom.
Fields in GraphicsStatsProto cannot be renamed as these were
part of previous Android releases.

Test: Ran "adb shell cmd stats pull-source 10068"
Test: Ran "statsd_testdrive 10068" and it looks OK
Bug: 149646555
Change-Id: Idc326eb2a7eb9ff0be39026e6fec544491b7d8a6