Testconfig: Set available android toolchain version

Android default toolchain version will be updated to 4.8, but ci is
only having version 4.7. Added configure option to use that until
nodes will get updated.

Change-Id: Ie7937d0c0c5e1caa609140f03c8e6f66f563985d
Reviewed-by: BogDan Vatra <bogdan@kde.org>
Reviewed-by: Heikki Halmet <heikki.halmet@digia.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>