[droid] clear the depth buffer after rendering an addon.
This fixes slow GUI after vis/screensaver. We don't use the depth buffer, but addons do. Prior to this change, we were relying on them to clean up after themselves. They weren't, and it led to a ~25% gui slowdown after they'd started. TODO: We should be pushing lots more settings to our state block. A poorly written vis/ss could potentially mess with much of our state.