This PR removes the MaterialApp widget's accentColor dependency per #56918.

The reference in question was used to set the GlowingOverscrollIndicator's color that is created for Material viewports. This PR updates this to now use the colorScheme.secondary for the overscroll indicator instead.

Breaking change

This is a breaking change, although most apps will not notice the change from theme.accentColor to theme.colorScheme.secondary, because they are typically the same color. For apps that do need a
specific color for the glow indicator, you can wrap your scrollable with something like:

    ScrollConfiguration(
      behavior: const ScrollBehavior(),
      child: GlowingOverscrollIndicator(
        axisDirection: AxisDirection.down,
        color: Colors.yellow,
        child: ListView.builder(
          ...
        ),
      ),
    ),

Which will force the glow color to yellow.

A new test was added to check verify the overscroll indicator color for Material apps.

This PR was tested against internal Google apps in cl/360771909