BlueprintDomainCombiner should not be holding a reference to BundleContext

The BlueprintProtectionDomain holds a reference to a bundle context which can become invalid during state changes. This can cause permission checks to fail when starting/stopping features or bundles when a security manager is being used because of the invalid context. Other ProtectionDomain implementations, such as the Felix one for ConfigAdmin hold a reference to the Bundle instead and seem to work as intended.