Cronet API: make UrlRequest.Callback an interface.

For consistency with other Callback classes, and not to force class
hierarchy on users. The historical "callback completeness" issue can be
mnitigate using interface defaults.

Test: atest CtsNetHttpTestCases
Change-Id: Ibf2aaf4396ab9defdeb4e3882cea6b1b2dbd36ee
Merged-In: Id8fcc13567da8878e848f3c6668d865e77b48146
3 files changed