Add package for projects/integrations/Unity (multiple)

Summary:
Our manifest merger is outdated, dating back to 2015.
To improve its functionality, we need to decouple it from AOSP and upgrade to the latest version.
Here's an initial attempt at doing so: D83352021
However, with the new manifest mergers, every library's AndroidManifest.xml should have a package field, which many of our repository's libraries currently lack.
We must address this issue before we can successfully land the diff which is what this stack of diff is about.
More: T239793438

Reviewed By: hick209

Differential Revision: D86437831

fbshipit-source-id: 6604faeec6bc256df2b37fda277cee83465c9911

Author: NavidQar

Assets/Plugins/Android/AndroidManifest.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="utf-8" standalone="no"?>
-<manifest xmlns:android="http://schemas.android.com/apk/res/android" android:installLocation="auto">
+<manifest xmlns:android="http://schemas.android.com/apk/res/android" android:installLocation="auto" package="com.meta.unity-startersamples">
   <application android:label="@string/app_name" android:icon="@mipmap/app_icon" android:allowBackup="false">
     <activity android:theme="@android:style/Theme.Black.NoTitleBar.Fullscreen" android:configChanges="locale|fontScale|keyboard|keyboardHidden|mcc|mnc|navigation|orientation|screenLayout|screenSize|smallestScreenSize|touchscreen|uiMode" android:launchMode="singleTask" android:name="com.unity3d.player.UnityPlayerActivity" android:excludeFromRecents="true" android:exported="true">
       <intent-filter>