flatpak: Add app to be uninstalled to txn cache

This mirrors what we do for install operations, and ensures that the
GsApp object used by the GsFlatpakTransaction will be the same one which
was passed to gs_plugin_app_remove(), which prevents this warning:

(/builds/mwleeds/gnome-software/_build/plugins/flatpak/gs-self-test-flatpak:1689):
Gs-WARNING **: 21:05:06.803: application
user/flatpak/chiron1-origin/desktop/org.test.Chiron/master left in
removing helper
2 jobs for misc-flatpak-fixes in 4 minutes and 13 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #723262
debian-stable-x86_64

00:02:42

passed #723261
fedora-x86_64

00:04:13