Support `android:taskAffinity=""` attribute

[kjm0202]

[REQUIRED] Step 1: Describe your environment

Plugin Version

6.0.0

[REQUIRED] Step 2: Describe the problem

This issue is same with this issue (with video): #1322 (comment)
But, I think it shoud rather be opened than closed, as it is not fixed yet.
Flutter's default setting is android:taskAffinity="", and flutter_mobile_ads package should follow this.
Just removing this attribute might cause any security issue or programmatic issue.

• It should be documented anywhere for developers.

Steps to Reproduce

1. Run any AdMob example
2. Click the ad - redirected to Ad page.
3. Press back button

Expected results:
Go back to my app

Actual results:
Just goes to device's home.
There's dead, blank activity on device's task manager.

Logs

[malandr2]

Thanks @kjm0202, I'll have @LTPhantom take a closer look to determine next steps.