ImageDecoder: use kYes_ZeroInitialized memory

Bug: 183115528
Test: (A)ImageDecoderTest(s) (verify correctness)
Test: monitor showmap_pss_bytes dashboards (verify memory impact)

This lets the decoder leave zero initialized memory untouched. An
Android feature makes untouched zero initialized memory cheaper, and
BitmapFactory takes advantage of it. Do the same for ImageDecoder.

This feature was originally brought up in b/10016979. ImageDecoder saved
memory in other ways, and as I understand it, Android has shifted
towards using (Animated)VectorDrawables. Both of these may have
contributed to us not noticing when we switched from BitmapFactory to
ImageDecoder.

Change-Id: Iecfd1bbfdcc38e1f0bf380b4f4ea5b861cfcf08a