fix(high contrast text): Fix the color calculation used in high contrast text

- Treats more near-gray text as grayscale, which more-aggressively
  pushes text to pure white or black.
- Text treated as colorful will now never worsen the contrast by
  lightening already-dark or darkening already-light colors.

Also adds unit tests for this calculation.

NO_IFTTT=refactoring

Bug: 384793956
Flag: com.android.graphics.hwui.flags.high_contrast_text_inner_text_color
Test: See screenshot in b/384793956 comment24
Test: atest DrawTextFunctorTest
Change-Id: I9efa13f1741ac45e095e990952a45fdeaa9dce30