fix: update location of default UnityManifest.xml

[manugildev]

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Pull Request Details

Unity renamed the default AndroidManifest.xml to UnityManifest.xml in 2019.3. This hasn't been updated in this repo, what causes issues extra steps when trying to configure the Facebook SDK, specially in Unity-6 with the new GameActivity application entry.

Test Plan

Test Plan:

1. Enable GameActivity as ApplicationEntry in your Android PlayerSettings
2. Add facebook sdk to your project
3. Click Regenerate Android Manifest on the Facebook Settings
4. See that the generated manifest inside Plugins/Android is based on the old facebook provided DefaultAndroidManifest.xml, instead of the one coming from Unity.
5. Running the app will fail with error: MAIN + LAUNCHER not found in any activity of your AndroidManifest

After this PR, the issue does not happen.