debian/*: fix foreign-arch install of us and native lomiri-app-launch

Since 9012bd4e ("session-migration: migrate LUITK's theme config"), we've been breaking the cross-installation of ubuntu-sdk-libs. Turns out, the root cause is us requiring foreign session-migration vs. native lomiri-app-launch requiring native session-migration.

lomiri-app-launch itself requires click-apparmor, which due to python3 dependency can't be installed foreign either, which means it's us that has to drop the requirement on foreign session-migration. To do this, move the migration script & the session-migration dependency to a separated -common package, which we'll make 'Architecture: all' and 'Multi-Arch: foreign' to make it native-installable.

With this, apt should consider -common native, and subsequently installs native version of session-migration. Along with native version of lomiri-app-launch, this should fix installation of foreign ubuntu-sdk- libs.

Fixes: 9012bd4e ("session-migration: migrate LUITK's theme config")