TextClassifierImpl: Handle urls.

This cl also updates label strings, renames an internal method,
and fixes some bugs in TextClassifierImpl caught by tests.

Test: See: I31ee94177e58c986e61e9b24c6c5816fa4b022e1
Bug: 34778263
Bug: 34922928
Change-Id: Ibf60d345d462dfc8a13dead6f01053f1ae50f35a