Migrate helloworld to use {usesCleartextTraffic} manifest placeholder

Summary:
This removes the need to specify 2 Manifests for apps and we can just use the `main` manifes to toggle if `usesCleartextTraffic` should be enabled or not.

This will have to be replicated in the template repository.

Changelog:
[Android] [Added] - Add support to specify a single Manifest rather than 2 (main/debug) by using the `usesCleartextTraffic` manifest placeholder which is autoconfigured by RNGP.

Differential Revision: D78425139

Author: cortinico

private/helloworld/android/app/src/debug/AndroidManifest.xml

@@ -8,6 +8,7 @@
       android:icon="@mipmap/ic_launcher"
       android:roundIcon="@mipmap/ic_launcher_round"
       android:allowBackup="false"
+      android:usesCleartextTraffic="${usesCleartextTraffic}"
       android:theme="@style/AppTheme">
       <activity
         android:name=".MainActivity"