Default fake gamma to sRGB.

No one appears to be using the default 2.2.
This makes SK_GAMMA_SRGB the default / a no-op, which seems more realistic.

Chrome's explicitly setting SK_GAMMA_EXPONENT or SK_GAMMA_SRGB.
We set Android platform explicitly to 1.4.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2198073002

Review-Url: https://codereview.chromium.org/2198073002
3 files changed