Define HostInstance type for React Native (#46743)

Summary:
Pull Request resolved: #46743

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`.

Changelog:
[General][Added] - Added `HostInstance` type to represent the instance of a `HostComponent<T>`.

Test Plan:
Ran the following successfully:

```
$ cd ~/fbsource
$ js1 flow
```

Reviewed By: NickGerleman

Differential Revision: D63646555

Pulled By: yungsters

fbshipit-source-id: 075859e6b0217521ba9ab2b92843ae7e47db5be0

packages/react-native/Libraries/Renderer/shims/ReactNativeTypes.js

@@ -7,7 +7,7 @@
  * @noformat
  * @nolint
  * @flow strict
- * @generated SignedSource<<b803401b6dd721b9caffdac1f8b6fd1c>>
+ * @generated SignedSource<<9e6c8931d3b0c36d35ad5da90b721b85>>
  */
 
 import type {
@@ -113,31 +113,32 @@ 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,
@@ -210,7 +211,7 @@ 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,
@@ -219,14 +220,11 @@ 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,
@@ -248,20 +246,17 @@ 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,