Soong: Add synopsis to soong_namespace module.

Synopsis was missing to soong_namespace module. Also added documentation
to imports properties. This required the namespaceProperties struct to be
pulled out of the NamespaceModule struct in order for the documentation
tool to extract the property documentation.

Bug: b/128337482
Test: Ran the doc generation command and verified that the synopsis was
added to soong_namespace and imports properties is listed too.

Change-Id: I519b14629bdbc85f35fbc8fa03e78dc2ad3f655e