Cache checkbox locally

[PureWeen]

Description of Change

When we initially did checkbox, we didn't cache the images used for disabled because it would always tint them grey. Well, now it seems to always tint them gray no matter how many different StackOverflow posts I found. So, I modified the code to not recreate the images every time and cause a layout loop. I logged the issue here about the tinting.

The code for testing this issue is

MainPage = new ContentPage
{
	Content = new StackLayout()
	{
		Children =
		{
			new CheckBox() { Color = Colors.Purple },
			new CheckBox() {  Color = Colors.Purple, IsEnabled = false },
		}
	}
};

Which is another disappointment with this PR :-) When placed in our test runners this code runs fine and doesn't cause a layout loop.

• I removed a bunch of internal methods that weren't used anywhere
• I made the exception for a missing handler more clear. This came up as I was attempting to write a unit test

Issues Fixed

Fixes #6687

[SotoiGhost]

LGTM! 👍🏼