Cache ShellImpl by resolved URL, not initial URL

This prevents us from loading applications twice when URLs are mapped.

BUG=427974
R=davemoore@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/8c71ea8f07989d102ea575f4d75947392900c414

Review URL: https://codereview.chromium.org/696563002