Introduce the class AndroidQmakeBuildConfiguration whose purpose is
to set the environment variable. Modify the Factory to create
buildconfigurations of that type and fix restore/clone to also
take the factories' priorities into account.

Change-Id: Icb377fa9211cd3564c36b60cf7c5f7dd84fcab50
Reviewed-by: BogDan Vatra <bogdan@kde.org>