Verify that Uri without scheme throws proper Exception

Bug: 154954875
Bug: 143231863
Test: This

I9229039b752bafa9a9b85b914a62093dff1eec34 switched from "==" to
".equals", but it did not take into account that Uri#getScheme can
return null. Verify that a Uri with a null scheme (which is otherwise
invalid) throws a FileNotFoundException instead of a
NullPointerException.

Change-Id: I93b01cb60a8c76110ea2ff4250d0048512629d8d