rename unstable_Offscreen to unstable_Activity (#27640)

Summary:
`Activity` is the current candidate name. This PR starts the rename work
by renaming the exported unstable component name.

NOTE: downstream consumers need to rename the import when updating to
this commit.

DiffTrain build for commit https://github.com/facebook/react/commit/ce2bc58a9f6f3b0bfc8c738a0d8e2a5f3a332ff5.

Reviewed By: tyao1

Differential Revision: D50945046

Pulled By: kassens

fbshipit-source-id: be9b3254c7a98840b0769135770e9bf7858cf1a3

packages/react-native/Libraries/ReactNative/renderApplication.js

@@ -22,7 +22,7 @@ import * as React from 'react';
 // require BackHandler so it sets the default handler that exits the app if no listeners respond
 import '../Utilities/BackHandler';
 
-type OffscreenType = React.AbstractComponent<{
+type ActivityType = React.AbstractComponent<{
   mode: 'visible' | 'hidden',
   children: React.Node,
 }>;
@@ -73,13 +73,13 @@ export default function renderApplication<Props: Object>(
   if (useOffscreen && displayMode != null) {
     // $FlowFixMe[incompatible-type]
     // $FlowFixMe[prop-missing]
-    const Offscreen: OffscreenType = React.unstable_Offscreen;
+    const Activity: ActivityType = React.unstable_Activity;
 
     renderable = (
-      <Offscreen
+      <Activity
         mode={displayMode === DisplayMode.VISIBLE ? 'visible' : 'hidden'}>
         {renderable}
-      </Offscreen>
+      </Activity>
     );
   }