keyboard: Stop using Shell.GenericContainer

keyboard: Stop using Shell.GenericContainer

This is the last remaining usage of Shell.GenericContainer
in the codebase, and posed small challenges compared to the
other removals.

A new St.Widget subclass called InputSourceIndicatorContainer
was added as a replacement to the Shell.GenericContainer. It
was needed because GNOME Shell needs to override the regular
size allocation functions, but InputSourceIndicator already
is a St.Widget with its own size allocation overrides.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/153