LengthGenerator is making the mistake of having a static reference
to java.lang.String without FQCN. This breaks down if String is
overridden by definition context.
Fix this by using importedName(Type) to emit a properly-scoped
reference.
JIRA: MDSAL-664
Change-Id: I16b12865448bde807f0afd0d35e3145f8b6359c3 Signed-off-by: Robert Varga <robert.varga@pantheon.tech>