Android: Make sign package orthogonal to the debug vs release

This enables signing debug packages and creating unsigned release
packages.

Task-number: QTCREATORBUG-10060
Change-Id: I57d094972b451538e8e9e5104e803e50a4ba2336
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>
Reviewed-by: BogDan Vatra <bogdan@kde.org>