Removed ListTile accentColor dependency

[HansMuller]

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

ListTile used the accentColor for title text and for leading/trailing icons, in selected:true tiles, when the ListTileTheme did not specify a selectedColor. The Color scheme's primary color is now used instead.

If the ListTileTheme doesn't specify a non-null value for selectedColor, the new default icon colors for the tile's leading and trailing widgets, and the default text color for the titles' descendant text widgets:

brightness	new (selected:true)	old (selected: true)
light	colorScheme.primary	primaryColor
dark	colorScheme.primary	accentColor

These colors should be indistinguishable from the original version when using one of the default themes.

The text and icon colors for selected:false tiles have not changed.

Breaking change

This is a breaking change, although most apps will not notice the change from theme.primaryColor to theme.colorScheme.primary, because they are typically the same color. Apps should use the ListTileTheme's selectedColor to override the default.

import 'package:flutter/material.dart';

class Sample extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: ListTileTheme(
        selectedColor: Colors.orange,
        child: Container(
          padding: const EdgeInsets.all(32),
          alignment: Alignment.center,
          child: Column(
            mainAxisSize: MainAxisSize.min,
            children: <Widget>[
              ListTile(
                selected: true,
                title: Text('selected'),
                leading: Icon(Icons.android),
                trailing: Text('trailing')
              ),
              SizedBox(height: 32),
              ListTile(
                selected: false,
                title: Text('unselected'),
                leading: Icon(Icons.android),
                trailing: Icon(Icons.android),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

void main() {
  runApp(
    MaterialApp(
      theme: ThemeData.light(),
      home: Scaffold(body: Sample())
    ),
  );
}

This PR was tested against internal Google apps in cl/360280942

[darrenaustin]

LGTM, but there are some failing tests.

[HansMuller]

There was a old dependency on accentColor in a dev/test_apps/stocks. I've removed it. That should clear up the failing test.