Finish blocking activity when it goes invisible.

This way we avoid potential stale string when the blocking activity is
re-surfaced.

Change-Id: Ife17ef765822797be87f49942e143656319d5b47
Fixes: 113126734
Test: tested on device
(cherry picked from commit fdfeeb087c4a5d50f2b33eb6a6f09c4b6ac933cb)
diff --git a/service/src/com/android/car/pm/ActivityBlockingActivity.java b/service/src/com/android/car/pm/ActivityBlockingActivity.java
index fd792a6..aabdb72 100644
--- a/service/src/com/android/car/pm/ActivityBlockingActivity.java
+++ b/service/src/com/android/car/pm/ActivityBlockingActivity.java
@@ -197,6 +197,14 @@
     }
 
     @Override
+    protected void onStop() {
+        super.onStop();
+        // Finish when blocking activity goes invisible to avoid it accidentally re-surfaces with
+        // stale string regarding blocked activity.
+        finish();
+    }
+
+    @Override
     protected void onDestroy() {
         super.onDestroy();
         if (mCar.isConnected() && mUxRManager != null) {