Add launcher logging of back button

Bug: 127848641
Test: adb shell setprop log.tag.UserEvent VERBOSE, then hit back
Change-Id: I020738280d2e51a8a192e577baf48c0b092f22f6
(cherry picked from commit df7d00328c907f92419300925ffbdfbb20169422)
diff --git a/protos/launcher_log.proto b/protos/launcher_log.proto
index 5518f09..4974dcb 100644
--- a/protos/launcher_log.proto
+++ b/protos/launcher_log.proto
@@ -106,7 +106,7 @@
   RESIZE_HANDLE = 8;
   VERTICAL_SCROLL = 9;
   HOME_INTENT = 10; // Deprecated, use enum Command instead
-  BACK_BUTTON = 11; // Deprecated, use enum Command instead
+  BACK_BUTTON = 11;
   QUICK_SCRUB_BUTTON = 12;
   CLEAR_ALL_BUTTON = 13;
   CANCEL_TARGET = 14;
@@ -114,6 +114,7 @@
   SPLIT_SCREEN_TARGET = 16;
   REMOTE_ACTION_SHORTCUT = 17;
   APP_USAGE_SETTINGS = 18;
+  BACK_GESTURE = 19;
 }
 
 enum TipType {