Add support to flatpak sandbox: test feature

[uazo]

the patch is the author's original patch with minor modifications to allow the build without changes of other platforms.

see #1053 (comment)

source:

• https://github.com/flathub/io.github.ungoogled_software.ungoogled_chromium/blob/master/patches/chromium/flatpak-Add-initial-sandbox-support.patch#L891
• https://github.com/flathub/org.chromium.Chromium/blob/master/patches/chromium/flatpak-Add-initial-sandbox-support.patch

not used in:

• https://github.com/flathub/com.google.Chrome

Description

Please explain here what feature or bugfix these changes are addressing and why they should be included

All submissions

• there are no other open Pull Requests for the same update/change
• Cromite can be built with these changes
• I have tested that the new change works as intended (AVD or physical device will do)

Format

• patch subject and filename match (e.g. Subject: Alternative cache (NIK-based) -> Alternative-cache-NIK-based.patch)
• patch description contains explanation of changes
• no unnecessary whitespace or unrelated changes