Enable two way syncing for DriveSync

[andybak]

Example code from DriveSync.cs:

folderSyncs.Add(AddSyncedFolderAsync(
    "Videos",
    App.VideosPath(),
    deviceRootId,
    SyncType.Upload,
    SyncedFolderType.Videos,
    token,
    excludeExtensions: new[] { ".bat", ".usda" }));

All that's needed is to change SyncType to UploadAndDownload

I tested and it works. But:

1. We should probably not change this without adding a UI toggle and defaulting to "off"
2. How do we trigger a sync? Polling? A manual button to click? (See calls to SyncLocalFilesAsync for how it's currently triggered)
3. As an extra bit of security - change from excludeExtensions to includeExtensions?

[andybak]

For the record I've enabled this in the Plugin Scripting branch as it's the only way to easily edit scripts on a PC and get them on to a Quest.

[TimAidley]

IIRC the reason we only did the syncing one way in Tilt Brush was that we were worried about edge cases to with how to handle sketches being altered in more than one place etc.