We report download progress to UI when downloading outputs from remote cache. UI thread keeps track of active downloads. There are two cases the UI thread could leak memory:

1. If we failed to close the output stream, the `reporter.finished()` will never be called, prevent UI thread from releasing the active download. This is fixed by calling `reporter.finished()` in `finally` block.
2. Normally, UI thread stops after `BuildCompleted` event. However, if we have background download after build is completed, UI thread is not stopped to continue printing out download progress. But after all downloads are done, we forgot to stop the UI thread, resulting all referenced objects leaked. This is fixed by calling `checkActivities()` for every download progress.

Fixes bazelbuild#18145.

Closes bazelbuild#18593.

PiperOrigin-RevId: 539923685
Change-Id: I7e2887035e540b39e382ab5fcbc06bad03b10427