ImageDecoder: throw FileNotFoundException on null

Bug: 140961740
Test: I41a93b47acde6a7985c53107f448a8b647d245d7

If ImageDecoder has a null AssetFileDescriptor, either from a Callable
or from a ContentResolver directly, throw a FileNotFoundException.
Previously, a NullPointerException was thrown attempting to dereference
it.

Change-Id: Ie738b9edc062e520835010befc001578fce09832
1 file changed