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
diff --git a/libs/hwui/protos/graphicsstats.proto b/libs/hwui/protos/graphicsstats.proto
index dd5676c..745393c 100644
--- a/libs/hwui/protos/graphicsstats.proto
+++ b/libs/hwui/protos/graphicsstats.proto
@@ -30,8 +30,9 @@
 
 message GraphicsStatsProto {
     enum PipelineType {
-        GL = 0;
-        VULKAN = 1;
+        UNKNOWN = 0;
+        GL = 1;
+        VULKAN = 2;
     }
 
     // The package name of the app