Use host URL, including scheme, for companion download URL

[ewpatton]

Change-Id: I49c1cef27b8daa226479e2b9cc6a609a43674e7c

General items:

• [ ] I have updated the relevant documentation files under docs/
• [x] My code follows the:
  • [x] Google Java style guide (for .java files)
  • [ ] Google JavaScript style guide (for .js files)
• [x] ant tests passes on my machine

For all other changes:

• [x] I branched from master
• [x] My pull request has master as the base

What does this PR accomplish?

This PR uses the base URL for computing the companion download link in the About the Companion dialog. This fixes #3190 by ensuring that HTTPS downloads of the companion are respected with App Inventor is served over HTTPS. On Chrome, using the HTTP URL when serving over HTTPS will prevent the download from happening.