ultrahdr: update error checks in encode/decode path

1. library assumes the gain map resolution is 1/kMapDimensionScaleFactor
of primary image resolution. If it is not the case the output hdr image
computations are not according to the scale factor. Signal error if map
scale factor is not kMapDimensionScaleFactor.
2. library expects primary image to be 420 and gain map image to be 400.
Validate the same.
3. disallow srgb transfer function way early as this is not handled.
4. avoid nullptr access in metadata.

Bug: 284120011
Test: ./ultrahdr_enc_fuzzer
Change-Id: Ie2cd12ac95306958cbb18679039681fc4eb3d9cc