docs: Document necessary android:taskAffinity option to make Android AppAuth work

[Vortex375]

Description

I had difficulties getting this package to work on android. After following the "Getting Started" instructions step-by-step I was unable to successfully authenticate on Android. I kept getting the error message

AppAuth: No stored state - unable to handle response

After some extensive Googling I found this comment on GitHub: MaikuB/flutter_appauth#503 (comment)

It tells you to remove the empty property android:taskAffinity="" from AndroidManifest.xml. This instantly fixed the issue for me.

I did not add this empty property, so it seems that flutter now creates this empty property by default. Therefore, it should be documented in the "Getting Started" steps for Android, that this property needs to be removed.

A user in the other GitHub thread mentions that instead of removing this property, it can also be set to your appId, e.g. android:taskAffinity="com.examle.app". I don't know what difference it makes though, so I don't know which option should be documented in the wiki here.

[ahmednfwela]

We have plans to completely get rid of the AppAuth dependency, which will open up the door for us to implement more advanced functionalities