iOS version of #149033.

Web version implemented with #134501.

Original issue

This is like #134501, but for mobile.

A use case for addtoapp would be to embed Flutter into a native scrollable of some sort. Consider the following:

At this time, on mobile platforms, the size of the Flutter view must be fixed in the embedding. This reduces the design space for the embedded Flutter app.

<LinearLayout
  android:id="@+id/flutter_view_container"
  android:layout_width="match_parent"
  android:layout_height="500dp" />

Instead, we want to be able to set wrap_content:

<LinearLayout
  android:id="@+id/flutter_view_container"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"/>

See b/342534475 for more internal context.