Fix wrong time unit in dev server OKHttp client. (#19817)

Summary:
The keep-alive-duration for the connection pool for mOnChangePollingClient
is a number in milliseconds, but was mistakenly given in minutes. This mistake
was introduced in 6bbaff2944.

I discovered this by chance while lurking around in the codebase.
Pull Request resolved: https://github.com/facebook/react-native/pull/19817

Differential Revision: D8873138

Pulled By: hramos

fbshipit-source-id: c81c1952721449442f18a0c2373f8b2780403749

ReactAndroid/src/main/java/com/facebook/react/devsupport/DevServerHelper.java

@@ -539,7 +539,7 @@ public class DevServerHelper {
     mOnChangePollingEnabled = true;
     mOnServerContentChangeListener = onServerContentChangeListener;
     mOnChangePollingClient = new OkHttpClient.Builder()
-        .connectionPool(new ConnectionPool(1, LONG_POLL_KEEP_ALIVE_DURATION_MS, TimeUnit.MINUTES))
+        .connectionPool(new ConnectionPool(1, LONG_POLL_KEEP_ALIVE_DURATION_MS, TimeUnit.MILLISECONDS))
         .connectTimeout(HTTP_CONNECT_TIMEOUT_MS, TimeUnit.MILLISECONDS)
         .build();
     enqueueOnChangeEndpointLongPolling();