fix DownloadDialog crash if there are no streams available yet

[evermind-zz]

If there are no streams the DownloadDialog will close again quickly, but the rxJava job fetching the VideoSegments is still going. This patch fixes that:

• move initialization of getting VideoSegments rxJava job until after initToolbar()
• have Disposable for the VideoSegments stuff that will be disposed if the DownloadDialog will be destroyed.

What is it?

• Bugfix (user facing)
• Feature (user facing)
• Codebase improvement (dev facing)
• Meta improvement to the project (dev facing)

Description of the changes in your PR

fix DownloadDialog crash if there are now streams available yet

If there are no streams the DownloadDialog will close again quickly,
but the rxJava job fetching the VideoSegments is still going. This patch
fixes that:
- move initialization of getting VideoSegments rxJava job until after initToolbar()
- have Disposable for the VideoSegments stuff that will be disposed if the
  DownloadDialog will be destroyed.

APK testing

The APK can be found by going to the "Checks" tab below the title. On the left pane, click on "CI", scroll down to "artifacts" and click "app" to download the zip file which contains the debug APK of this PR. You can find more info and a video demonstration on this wiki page.

Due diligence

• I read the contribution guidelines.