Remove IPlatformServices.RequestedTheme and use Essentials

[mattleibow]

Description of Change

The logic for the app theme is split up a bit, Essentials just reports the current theme and Core handled the change events on the views.

This PR merges all the ideas and removes the redundancy. Device.RequestedTheme just uses the Essentials API, and when Core decides it is time to trigger a theme change event, it fires the event and then Controls puts it all together.

Part of #1965

Additions made

• Removes IPlatformServices (finally)
• Removes OSAppTheme in favour of the exactly the same AppTheme from essentials

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)
• Targets a single property for a single control (or intertwined few properties)
• Adds the property to the appropriate interface
• Avoids any changes not essential to the handler property
• Adds the mapping to the PropertyMapper in the handler
• Adds the mapping method to the WinUI, Android, iOS, and Standard aspects of the handler
• Implements the actual property updates (usually in extension methods in the Platform section of Core)
• Tags ported renderer methods with [PortHandler]
• Adds an example of the property to the sample project (MainPage)
• Adds the property to the stub class
• Implements basic property tests in DeviceTests

Does this PR touch anything that might affect accessibility?

• Does this PR introduce a new control? (If yes, add an example using SemanticProperties to the SemanticsPage)
• APIs that modify focusability?
• APIs that modify any text property on a control?
• Does this PR modify view nesting or view arrangement in anyway?
• Is there the smallest possibility that your PR will change accessibility?
• I'm not sure, please help me

If any of the above checkboxes apply to your PR, then the PR will need to provide testing to demonstrate that accessibility still works.