[Android] Keep authentication on the same activity stack

[Mino5531]

This PR changes the authentication process on Android, so that the chrome custom tab window opens on the same activity stack as the flutter application and no longer opens a new activity stack or window. This should improve end user experience as it is no longer possible for users that leave the application, to copy a 2FA code for example, to return to the wrong "window"

This all is achieved by starting a new activity (in this case AuthenticationManagementActivity) before starting the chrome custom tab. The CallbackActivity will then start the AuthenticationManagementActivity which will then finish and return to the original flutter activity.

Breaking changes
Applications must register a new activity:

<activity android:name="com.linusu.flutter_web_auth_2.AuthenticationManagementActivity">
</activity>

[ThexXTURBOXx]

Thank you very much for your contribution! I have looked through it and it works wonders! :)
Will publish this today as part of a 5.x alpha