[Autofill Assistant] Record unexpected taps as OVERLAY_STOP.

Before this change, both unexpected taps and cancels were recorded as
SHEET_CLOSED. This was wrong, as unexpected taps should be recorded as
OVERLAY_STOP.

Change-Id: I6f368611ecaf541fe70bc7a81195c91ada6da116
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1726089
Auto-Submit: Stephane Zermatten <szermatt@chromium.org>
Reviewed-by: Jordan Demeulenaere <jdemeulenaere@chromium.org>
Commit-Queue: Jordan Demeulenaere <jdemeulenaere@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682269}

chrome/browser/android/autofill_assistant/ui_controller_android.cc

@@ -639,7 +639,7 @@ void UiControllerAndroid::OnUnexpectedTaps() {
   ShowSnackbar(l10n_util::GetStringUTF8(IDS_AUTOFILL_ASSISTANT_MAYBE_GIVE_UP),
                base::BindOnce(&UiControllerAndroid::Shutdown,
                               weak_ptr_factory_.GetWeakPtr(),
-                              Metrics::DropOutReason::SHEET_CLOSED));
+                              Metrics::DropOutReason::OVERLAY_STOP));
 }
 
 void UiControllerAndroid::UpdateTouchableArea() {