Add findVariantLink to StubUserLang

Add the full signature of Language::findVariantLink to StubUserLang so
that pass-by-reference variables are properly passed to the un-stubbed
Language object.

Bug: T78427
Change-Id: I51f57a17c814c43742f514df0e42a1760c406c48
Merge "Add findVariantLink to StubUserLang"