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/cmds/statsd/src/atoms.proto b/cmds/statsd/src/atoms.proto
index 23a4437..f215d77 100644
--- a/cmds/statsd/src/atoms.proto
+++ b/cmds/statsd/src/atoms.proto
@@ -8204,14 +8204,6 @@
 }
 
 /**
- * HWUI renders pipeline type: GL (0) or Vulkan (1).
- */
-enum PipelineType {
-    GL = 0;
-    VULKAN = 1;
-}
-
-/**
  * HWUI stats for a given app.
  */
 message GraphicsStats {
@@ -8224,9 +8216,16 @@
     // The start & end timestamps in UTC as
     // milliseconds since January 1, 1970
     // Compatible with java.util.Date#setTime()
-    optional int64 stats_start = 3;
+    optional int64 start_millis = 3;
 
-    optional int64 stats_end = 4;
+    optional int64 end_millis = 4;
+
+    // HWUI renders pipeline type: GL (1) or Vulkan (2).
+    enum PipelineType {
+        UNKNOWN = 0;
+        GL = 1;
+        VULKAN = 2;
+    }
 
     // HWUI renders pipeline type: GL or Vulkan.
     optional PipelineType pipeline = 5;