This greatly simplifies the post install script. As this functionality
depends on newer meson (0.59.0) it is guarded by a version check.
Distributions usually use their own tooling so it's does not matter if
they don't run the postinstall scripts (e.g. because of using too old
meson)
This makes the flatpak build and removes outdated manifest options
This also removes java since it was only needed to build the metadata
for libphonenumber which is already included in the source repo. See
https://github.com/google/libphonenumber/pull/2363