feat: Improve visibility of downloader

[oSumAtrIX]

Feature description

Right now the downloader is selectable through this:

The page needs to be redesigned to seamlessly offer the user to select a downloader. If the user selects an app they haven't installed yet, that dialog can appear. If dismissed, the user would go back to the app selector.

Motivation

In that screen its difficult to know what to do when you don't know beforehand that this button is necessary to be used.

Acknowledgements

• [X] This issue is not a duplicate of an existing feature request.
• [X] I have chosen an appropriate title.
• [X] The feature request is only related to ReVanced Manager