[Fix] exported in Android 12

[fabioselau077]

For people with problems still using Flutter, in addition to putting exported=true in the application's <activity, you need to put it in the <activity of the installed plugins.
That's the only way to work. It will look like this:
<activity android:name="clancey.simpleauth.simpleauthflutter.SimpleAuthCallbackActivity" android:exported="true">