Make ImageDecoder.setTargetColorSpace to EXTENDED promote to F16

Bug: 117601185
Test: I5e5bc6b73d5d27ae30f00507d59896008d4cf364

ImageDecoder does not have a way to request decoding to F16 on 8 bit
images, but it does have a way to request an EXTENDED ColorSpace. Treat
that as a request to use a higher bit depth. This matches how we match
EXTENDED to F16 in other cases (createBitmap, Bitmap#copy, etc).

Change-Id: I3d31b13b56bb57a23b47ffcb2eaba11c53e0b8d9