Decode file descriptor from the correct offset.

Fix a few bugs in nativeDecodeFileDescriptor:
1. Restore the FD's offset when exiting the function.
2. Copy the data when potentially using an SkImageRef.
   The old behavior would have continued to modify the
   file descriptor's offset each time a new decode was
   required. The copy ensures that the file descriptor
   remains unchanged.
3. Buffer the file stream.
   Prior to this change, if the image was not a PNG,
   the stream would be rewound to the beginning of the
   file, even if the file descriptor was passed in with
   an offset. Thanks to the buffer, the stream is only
   rewound to the original offset.

Depends on https://googleplex-android-review.googlesource.com/#/c/415821/1
in external/skia

BUG:12807677
BUG:12895876
Change-Id: I38b8cf5d210dbbc0107e6562e3884867de57fc4b