clarify that offset is not used in addPlatformView for iOS/Android

[RichardJCai]

Description
Currently addPlatformView's offset parameter is not used for iOS/Android. This PR clarifies that offset is not used for in addPlatformView for iOS and Android and removes offset from the scenario_app test for iOS/Android platform views.

Not yet removing offset completely since it may be used for Fuschia/Web. It may be desirable to remove offset completely for consistency between the platforms.

Also updated outdated comment stating addPlatformView is only supported for iOS.

Related Issues

flutter/flutter#37029

Tests

I added the following tests:
N/A

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[iskakaushik]

LGTM