Add a required main category for multi logs.

Test: updated LogBuilderTest

Change-Id: Ia5359e12a7a84af31d9a966f2c81fc943aadfbdb
diff --git a/proto/src/metrics_constants.proto b/proto/src/metrics_constants.proto
index 57c4565..d48dbeb 100644
--- a/proto/src/metrics_constants.proto
+++ b/proto/src/metrics_constants.proto
@@ -3175,6 +3175,11 @@
     // user accepted
     ACTION_SKIP_DISCLAIMER_SELECTED = 760;
 
+    // Enclosing category for group of APP_TRANSITION_FOO events,
+    // logged when we execute an app transition.
+    APP_TRANSITION = 761;
+
+
     // ---- End O Constants, all O constants go above this line ----
 
     // Add new aosp constants above this line.