Add high-brightness-mode range specifier to brightness mapping.

Within the device-configuration file, a device can specify a
point along the brightness mapping as the beginning of high-brightness
mode.  All points in the mapping beyond that range will be treated
as high-brightness-mode points.  Normal brightness range will end
at the specified point.

Bug: 131813802
Test: Verify brightness works through the entire pre-HBM range, with and
without new specifier.
Change-Id: I4d9c6fab1c1f11e550ee33643673b004ee54dbfe