Enable ios platform view by default

[cyanglaz]

Description

An app that uses platform view (webview, googlemap, etc) doesn't need to add the io.flutter.embedded_views_preview flag on their info.plist after this patch lands.

Related Issues

Fixes flutter/flutter#64270

Tests

I added the following tests:

Updated the scenario app to not include the flag and the tests should still pass

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[blasten]

Nice!

[blasten]

nit: should we just remove GetExternalViewEmbedderIfEnabled?

[cyanglaz]

This is the common method that's called by all the different surface implementations. So it is better to keep it there. Or we can update the method name (removing the IfEnabled)?

[blasten]

Yup. GetExternalViewEmbedder mirrors Android's.