[Fix] Download Offline Resources bugs

[CalebRas]

Description

This PR fixes some issues with the Download Offline Resources page:

• A crash when retrying failed downloads. e8190d4
• A bug where the "Download All" button would not re-enable if any downloads failed.
• A bug where multiple download-all operations would start when scrolling to and from the "Download All" button. 36363c5
• A bug where the "Download All" button's progress view would disappear after scrolling away from the button. 11be7c4
  • This was a SwiftUI bug that I worked around by making the download-all progress view also use GaugeProgressViewStyle.
• A hang when tapping "Done" after "Download All". 2f680d4
  • This appears to be an NSBundleResourceRequest bug that causes presentations to hang when downloading resources, even when downloading on a background thread. I could not find a way to work around this, so I removed the alert, which was confusing anyway. I updated the "Done" button show "Cancel" when there are in-progress downloads instead.
• A crash that occasionally happened during downloading.
  • This appears to be a race condition that would happen when a ProgressView accessed a dellocated Progress instance. To work around this, I changed OnDemandResource.progress to an object that gets manually updated from request.progress during downloading. 7c6b39b
• Some other minor code issues. 293450a

Linked Issue(s)

• swift/issues/7572

How To Test

Ensure the above-mentioned issues are no longer present.

• To simulate live downloads, set Embed Asset Packs In Product Bundle to No under the Target Build Settings.
• To simulate download errors, update the code to use invalid tags, such as by passing them in here, and try to download.

Screenshots

Before	After

[CalebRas]

There is one crash I didn't fix that appears to happen somewhere in the SwiftUI ProgressView implementation when it accesses a deallocated Progress object. It is sporadic at best, so I could not get it reproducing reliably enough to find a workaround. I included the stack trace in the issue if you are curious. (Fixed)

[CalebRas]

Hold off on reviewing this please. I may have found a fix for the Progress View crash.

[CalebRas]

There is one crash I didn't fix that appears to happen somewhere in the SwiftUI ProgressView implementation when it accesses a deallocated Progress object. It is sporadic at best, so I could not get it reproducing reliably enough to find a workaround. I included the stack trace in the issue if you are curious.

Fixed. The description has been updated, and this PR is ready for review again.