Add logging for bubble UI events.

Bug: 123543171
Test: manual on local device using statsd_testdrive
Change-Id: I4d344d94e39f52fdd743b99390616c23c86a3a45
diff --git a/cmds/statsd/src/atoms.proto b/cmds/statsd/src/atoms.proto
index 427662a..937588b 100644
--- a/cmds/statsd/src/atoms.proto
+++ b/cmds/statsd/src/atoms.proto
@@ -5263,6 +5263,7 @@
         PERMISSION_IGNORED = 12;
         SWIPE_LEFT = 13;
         SWIPE_RIGHT = 14;
+        STACK_EXPANDED = 15;
     }
     optional Action action = 6;