Define HostInstance type for React Native (#31101)

## Summary

Creates a new `HostInstance` type for React Native, to more accurately
capture the intent most developers have when using the `NativeMethods`
type or `React.ElementRef<HostComponent<T>>`.

Since `React.ElementRef<HostComponent<T>>` is typed as
`React.AbstractComponent<T, NativeMethods>`, that means
`React.ElementRef<HostComponent<T>>` is equivalent to `NativeMethods`
which is equivalent to `HostInstance`.


## How did you test this change?

```
$ yarn
$ yarn flow fabric
```

packages/react-native-renderer/src/ReactNativeTypes.js

@@ -112,31 +112,32 @@ export interface INativeMethods {
   measure(callback: MeasureOnSuccessCallback): void;
   measureInWindow(callback: MeasureInWindowOnSuccessCallback): void;
   measureLayout(
-    relativeToNativeNode: number | ElementRef<HostComponent<mixed>>,
+    relativeToNativeNode: number | HostInstance,
     onSuccess: MeasureLayoutOnSuccessCallback,
     onFail?: () => void,
   ): void;
   setNativeProps(nativeProps: {...}): void;
 }
 
-export type NativeMethods = $ReadOnly<{|
+export type NativeMethods = $ReadOnly<{
   blur(): void,
   focus(): void,
   measure(callback: MeasureOnSuccessCallback): void,
   measureInWindow(callback: MeasureInWindowOnSuccessCallback): void,
   measureLayout(
-    relativeToNativeNode: number | ElementRef<HostComponent<mixed>>,
+    relativeToNativeNode: number | HostInstance,
     onSuccess: MeasureLayoutOnSuccessCallback,
     onFail?: () => void,
   ): void,
   setNativeProps(nativeProps: {...}): void,
-|}>;
+}>;
 
 // This validates that INativeMethods and NativeMethods stay in sync using Flow!
 declare const ensureNativeMethodsAreSynced: NativeMethods;
 (ensureNativeMethodsAreSynced: INativeMethods);
 
-export type HostComponent<T> = AbstractComponent<T, $ReadOnly<NativeMethods>>;
+export type HostInstance = NativeMethods;
+export type HostComponent<Config> = AbstractComponent<Config, HostInstance>;
 
 type SecretInternalsType = {
   computeComponentStackForErrorReporting(tag: number): string,
@@ -209,7 +210,7 @@ export type RenderRootOptions = {
 export type ReactNativeType = {
   findHostInstance_DEPRECATED<TElementType: ElementType>(
     componentOrHandle: ?(ElementRef<TElementType> | number),
-  ): ?ElementRef<HostComponent<mixed>>,
+  ): ?HostInstance,
   findNodeHandle<TElementType: ElementType>(
     componentOrHandle: ?(ElementRef<TElementType> | number),
   ): ?number,
@@ -218,14 +219,11 @@ export type ReactNativeType = {
     child: PublicInstance | HostComponent<mixed>,
   ): boolean,
   dispatchCommand(
-    handle: ElementRef<HostComponent<mixed>>,
+    handle: HostInstance,
     command: string,
     args: Array<mixed>,
   ): void,
-  sendAccessibilityEvent(
-    handle: ElementRef<HostComponent<mixed>>,
-    eventType: string,
-  ): void,
+  sendAccessibilityEvent(handle: HostInstance, eventType: string): void,
   render(
     element: MixedElement,
     containerTag: number,
@@ -247,20 +245,17 @@ type PublicTextInstance = mixed;
 export type ReactFabricType = {
   findHostInstance_DEPRECATED<TElementType: ElementType>(
     componentOrHandle: ?(ElementRef<TElementType> | number),
-  ): ?ElementRef<HostComponent<mixed>>,
+  ): ?HostInstance,
   findNodeHandle<TElementType: ElementType>(
     componentOrHandle: ?(ElementRef<TElementType> | number),
   ): ?number,
   dispatchCommand(
-    handle: ElementRef<HostComponent<mixed>>,
+    handle: HostInstance,
     command: string,
     args: Array<mixed>,
   ): void,
   isChildPublicInstance(parent: PublicInstance, child: PublicInstance): boolean,
-  sendAccessibilityEvent(
-    handle: ElementRef<HostComponent<mixed>>,
-    eventType: string,
-  ): void,
+  sendAccessibilityEvent(handle: HostInstance, eventType: string): void,
   render(
     element: MixedElement,
     containerTag: number,