Decoder only returns true on complete success.

Recently we added a way for SkImageDecoder::decode to distinguish
between successfully decoding the entire image and only partially
decoding the image (see https://codereview.chromium.org/647023006).

Only consider a call to decode() a success if the image was completely
decoded. This matches pre-L behavior, and lets the caller know that
they need to try to decode again.

Requires a change to external/skia (I33e6940e247b74b20361ae041f8d36eb600df49f)

BUG:17419670
Change-Id: I17ed7288b2359fafaec9551adb16d1d037800eb7