The new splash screen implementation doesn't show the images for a simple-drawable splash, and doesn't reuse existing images when converting to a theme splash

We now have a new splash screen implementation that uses a theme-based splash, which looks better at application startup. There are usability issues with this, as mentioned in the summary:

1. The splash screen editor allows converting an existing manifest into a manifest that has the xml theme splashes, but while allowing that conversion, it doesn't provide a preview of the existing images.
2. The conversion just converts the manifest and drops the existing splash images. That seems unfortunate, as they are presumably of a suitable size and could just be reused.
3. The implementation doesn't allow selecting svg images any more. That's unfortunate, because svg images that have size hints would work fine.