Split loadApplicationScript into initializeRuntime and loadBundle (#27844)

Summary:
This is the first of three PRs related to enabling multi-bundle support in React Native. More details, motivation and reasoning behind it can be found in RFC [here](https://github.com/react-native-community/discussions-and-proposals/issues/152).

Logic responsible for installing globals was pulled out from `loadApplicationScript` to `initializeRuntime` since it should be ran only once, what was left was renamed to `loadBundle`.

It's based on dratwas work from [here](https://github.com/callstack/react-native/tree/feat/multibundle/split-load-application), but applied to current `master` to avoid rebasing 3-months old branch and issues that come with that.

## Changelog

[Internal] [Changed] - split `loadApplicationScript` into `initializeRuntime` and `loadBundle` to enable multi-bundle support in the future
Pull Request resolved: https://github.com/facebook/react-native/pull/27844

Test Plan: Initialized new RN app with CLI, set RN to build from source and verified the still app builds and runs OK using code from this branch.

Reviewed By: rickhanlonii

Differential Revision: D19888605

Pulled By: ejanzer

fbshipit-source-id: 24ace48ffe8978796591fe7c6cf53a61b127cce6

ReactAndroid/src/test/java/com/facebook/react/devsupport/JSDebuggerWebSocketClientTest.java

@@ -40,7 +40,7 @@ public class JSDebuggerWebSocketClientTest {
   }
 
   @Test
-  public void test_loadApplicationScript_ShouldSendCorrectMessage() throws Exception {
+  public void test_loadBundle_ShouldSendCorrectMessage() throws Exception {
     final JSDebuggerWebSocketClient.JSDebuggerCallback cb =
         PowerMockito.mock(JSDebuggerWebSocketClient.JSDebuggerCallback.class);
 
@@ -49,7 +49,7 @@ public class JSDebuggerWebSocketClientTest {
     injectedObjects.put("key1", "value1");
     injectedObjects.put("key2", "value2");
 
-    client.loadApplicationScript("http://localhost:8080/index.js", injectedObjects, cb);
+    client.loadBundle("http://localhost:8080/index.js", injectedObjects, cb);
     PowerMockito.verifyPrivate(client)
         .invoke(
             "sendMessage",