meta-app-framework: agl-session: Ensure XDG_DATA_DIRS is set

meta-app-framework: agl-session: Ensure XDG_DATA_DIRS is set

Add installation of a systemd user-environment-generator scriptlet
to the agl-session recipe to ensure that XDG_DATA_DIRS is set to a
reasonable default value.  This is required for locating D-Bus
activation .service files and icons.

Bug-AGL: SPEC-4182

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I5d1a72022ca97f25a915b64205bf70ab33516ec6
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/27048
Reviewed-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>