[Fabric] Expand native theming implementation

[acoates-ms]

Description

• Changed usage of platform colors/brushes to use a context aware Theme object allowing future tree specific theming and native theme injection.
• Added values to the platform colors for usage by Switch, TextInput and ActivityIndicator.
• Added a brush cache.
• Optimize rootComponentView lookup.
• Updated switch visuals to handle more visual states and use theme colors rather than hard coded values.
• Standardized more component border handling
• Modified CompActivityVisual, CompCaretVisual, and CompSwitchThumbVisual to take brushes instead of colors for expanded brush support
• Implemented ScrollItem and Value accessibility patterns
• Implemented IsOffScreen accessibility property
• Stopped creating automation peers internally until they are requested by a UIA client
• Fixed an issue where incorrect bounding rect would be returns by components within in a ScrollView that was scrolled to a non-zero offset
• Updated E2E test logic to use SetValue to clear the search textinput rather than entering backspaces
• Added retry logic to E2E test page navigation to improve reliability of E2E tests
• Fixed a crash when reloading the instance in the fabric playground app
• Fixed a crash when using mousewheel in non-islands fabric apps
• Fixed intermittent crash getting automation peer from a loading rootview
• Fixed issue in hittesting where items outside of the viewport of a ScrollView would still be hit tested.

Microsoft Reviewers: Open in CodeFlow

[chiaramooney]

Testing changes look good to me!