Explicitly state all File constructor NPEs.

I think I closed the previous bug report as "unless otherwise stated, you
should assume that passing null to anything gets you a NullPointerException",
but this is at least the second time this specific case has been reported as
a bug, so...

Bug: https://code.google.com/p/android/issues/detail?id=136551
Change-Id: I5993bbf405054f57cd4f7c05af38e6c9d66355e7