testing/signal-desktop: upgrade to 7.32.0, use default_prepare

https://github.com/signalapp/Signal-Desktop/releases/tag/v7.32.0

Also refactors to use default_prepare patching logics:

Aport had previously implemented custom patching logics. This is
redundant, thus this refactor removed these custom logics by moving the
source code that was outside of builddir so that default_prepare could
be used. All patches were rebased for this