Do not resolve caller_ns when it is not needed

create_namespace resolves caller namespace only
when caller did not explicitly specify parent namespace.

This saves about 25 microseconds for the case when
parent_namepsace is not null (for example when creating
second classloader for the app).

Bug: http://b/28801010
Change-Id: I50ded272c931db701e5a1d8c88ed5ffb13416539