Check for null fd when constructing AssetFileDescriptor.

An AssetFileDescriptor with a null fd is pretty useless and should
never happen during normal operation.  It can't even be parcelled
without hitting an NPE.  To make it easier to find bugs where a null
fd might have been generated, check at construction time.

Bug: 5655036
Change-Id: I1da6dbee0cab9c11e309f3cebe698bf44dc5e27d