Push HLG OOTF down to libtonemap.

Usage of current display brightness may be vendor-configured when the
display brightness is very low, so keep the OOTF in libtonemap as part
of the reference implementation.

Concretely, this means that:
* The BT2100 recommended OOTF for HLG->output format is moved from
  ScaleLuminance in libshaders to be the first part of the tonemapping
  operator in libtonemap
* The inverse OOTF for input format->HLG is moved from
  NormalizeLuminance in libshaders to the end of the tonemapping operator
  in libtonemp
* Current display brightness is only taken into account in the default
  tonemapping for Android T. The historic tonemapper does not take into
  account current display brightness, as it treats the "nominal peak
  brightness" of the display as 1000 nits instead of the current
  brightness.

Also add a default lower-bound for using the current display brightness,
because not having a bound looks really terrible on existing shipping
devices

Bug: 208933319
Test: builds
Test: HLG test video looks okay
Test: HDR10 test video didn't break
Change-Id: I4f489c68f635a8ecc4d497b98c32e91c297d0765