RN: Revamp Switch Component

Summary:
Revamps the Switch API with the goal of increasing the pit of success:

- Introduce `trackColor` which encourages callers configuring the color to set colors for both cases.
- Introduce `ios_backgroundColor` which allows customizing the iOS-only background fill color.
- Deprecate `tintColor` because it is not obvious that this is for the `false` case.
- Deprecate `onTintColor` because the prop is named unconventionally like a callback.
- Renamed `thumbTintColor` to `thumbColor`.

This revision also cleans up the `Switch` component in the following ways:

- More precise Flow types for native components.
- Inline iOS-specific style (so that the code gets stripped on Android).
- Minor documentaiton cleanup.

After this commit, all deprecated props will continue working.

Next, I plan to introduce warnings.

Eventually (e.g. in a couple releases), we can drop support for the deprecated props.

Reviewed By: TheSavior

Differential Revision: D9081343

fbshipit-source-id: c5eb949047dd7a0ffa72621839999d38e58cada8

Libraries/Types/CoreEventTypes.js

@@ -80,3 +80,9 @@ export type ScrollEvent = SyntheticEvent<
     zoomScale: number,
   |}>,
 >;
+
+export type SwitchChangeEvent = SyntheticEvent<
+  $ReadOnly<{|
+    value: boolean,
+  |}>,
+>;