Convert proto3 to proto2 to avoid forced zero enum value.

The rationale for this change:
1. When defining enum values for platform, we want to use the current
integers, in some cases zero is not defined, but proto3 enforces a zero
default value.

2. Android Metrics Team uses proto2 on server-side

3. When copying .proto to server-side, the known issue of dropping
unknown fields might affect if using proto3

4. Not much benefits from using proto3

Bug: 67110257
Test: manully generate incident report and it looks normal
Change-Id: Ia63e39de549a46683e9f80fcb74f1d771782b7f4
diff --git a/core/proto/android/app/window_configuration.proto b/core/proto/android/app/window_configuration.proto
index 03910df..4d748e8 100644
--- a/core/proto/android/app/window_configuration.proto
+++ b/core/proto/android/app/window_configuration.proto
@@ -14,8 +14,7 @@
  * limitations under the License.
  */
 
-syntax = "proto3";
-
+syntax = "proto2";
 option java_package = "android.app";
 option java_multiple_files = true;
 
@@ -25,7 +24,7 @@
 
 /** Proto representation for WindowConfiguration.java class. */
 message WindowConfigurationProto {
-  .android.graphics.RectProto app_bounds = 1;
-  int32 windowing_mode = 2;
-  int32 activity_type = 3;
+  optional .android.graphics.RectProto app_bounds = 1;
+  optional int32 windowing_mode = 2;
+  optional int32 activity_type = 3;
 }