Fix leaving blank Activity on back stack.

When you use the "Erase and Open" shortcut to open the app, the blank EraseAndOpenShortcutActivity should not remain on the back stack.

app/src/main/java/org/mozilla/focus/activity/EraseAndOpenShortcutActivity.kt

@@ -23,5 +23,7 @@ class EraseAndOpenShortcutActivity : Activity() {
         intent.action = MainActivity.ACTION_OPEN
         intent.flags = Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_CLEAR_TOP
         startActivity(intent)
+
+        finish()
     }
 }