Several fixes to recovery map math.

* Fix incorrect conversion conversion from RGB to YUV using bt.2100 when
  the data is actually assumed to be srgb (although this will get
  improved further soon) in map application.
* Fix incorrectly calculating luminance directly from luma in map
  generation; we need to convert from YUV to RGB first. This also
  involved updating the sampling functions to sample entire pixels.
* Some cleanup to structure of recoverymapmath files.
* Call all public lib functions in recovermap_test to force linking, and
  therefore verify that the lib actually builds properly.

Test: builds (including the test, which verifies linking now)
Bug: 252835416
No-Typo-Check: incorrectly interpretting code as a comment
Change-Id: I13565145dc6efcdf642981a1e52420d662152737