Add support for exporting Meta's prebuilt apks

[m4gr3d]

The first commit adds support for exporting Meta's apks when using the Android / XR editor:

• Update the project to generate Meta's specific prebuilt apks.
  • This is done by adding Godot as a submodule to this project and updating the build scripts to generate the prebuilt apk using that submodule
  • The resulting prebuilt apks are stored in the addon and used by the export logic to override the custom templates, custom_template/debug and custom_template/release, options
• Leverage EditorExportPlugin#_update_android_prebuilt_manifest(...) introduced in Updates and fixes to the Android prebuilt export logic godotengine/godot#103173 to hook into the editor export logic for prebuilt apks, and modify prebuilt's manifest to match the project settings
  • This is only enabled on the Android / XR editor. On desktop, the plugin falls back to the gradle build

The second commit adds a gdextension configuration for the Android / XR editor:

• The previous logic only uses ANDROID_ENABLED to gate the Platform SDK logic, which causes it to be active when the addon is loaded in the Android / XR editor. The updated logic adds an editor specific build for the Android / XR editor which disables the Platform SDK logic.

Depends on godotengine/godot#103173

[dsnopek]

Currently, this PR uses a git submodule to point to the Godot source code for making the prebuilt APKs, but I think we'll probably want to be able to build multiple versions of Godot, so that folks can use an export template matching their editor version.

While I guess the editor plugin part will only work on Godot 4.5+ (since it needs it to modify the binary version of the AndroidManifest.xml), I could imagine that a single version of this extension could work across multiple Godot versions (for example, Godot 4.5 and 4.6) and we'll want to have builds for each of those versions.

I think this is more evidence for the idea that we probably don't want to include the export templates in the addon itself, but provide an editor tool to download them. (I also think this is a good idea, because it's mostly folks using the Android editor who will want to do the APK build, whereas folks working on desktop may be happy to keep doing the Gradle build, so we don't need to balloon the addon size for everyone.)