Make Half class use public APIs

Half has a dependency on an internal sun.misc.FloatingDecimal
that can be replaced by an equivalent call on java.lang.Float
(which calls through to FloatingDecimal).

Any performance hit is worth it for a smaller API surface.

Bug: 111055375
Test: Build
Change-Id: Iecdf3aa9414922a77edbdc439b0c2b88033b3af8