Only use the gainmap shader if we might need it

Skip using the gainmap shader when drawing to a bitmap
with a known SDR colorspace as it'll be a no-op in
rendering results since W=1.0, but it'll be very expensive
in runtime due to CPU evaluation of the SKSL shader.

Before:
createScaledBitmapWithGainmap_median (ns): 179,455,037

After:
createScaledBitmapWithGainmap_median (ns): 15,643,742

Bug: 311085927
Test: benchmark in CL
Change-Id: I3f2506a2bd86bbe29c6e0837d31e4a231a7c182a