Add descriptor validation to ClassLinker::FindClass().

And add tests for odd signatures passed to JNI GetFieldID().
Previously, passing the signature "java/lang/String" instead
of "Ljava/lang/String;" would call the class loader with the
dot name java.lang.String and the class loader would find
that class only to fail the DescriptorEquals() check back
in ClassLinker::FindClass().

Test: 647-jni-get-field-id
Bug: 33577836
Bug: 37156832
Change-Id: I6612a272ec24b0d54b728fd35003e9c24a7e2e95