flatpak: Build commands as lists instead of strings (#269)

Using a list ensures that all subprocess arguments are correctly
escaped. By building strings and then calling .split(), potential
arguments with a space will be incorrectly split over two arguments.
When a string is needed for presentation, join to the list to build the
string.

changelogs/fragments/269-flatpak-command-list.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+- flatpak and flatpak_remote - fix command line construction to build commands as lists instead of strings.