Add a half-precision floating point type

Denormals are fully supported.

This will be useful to store wide-gamut colors in a compact form
(a full RGBA color can be stored in a long using half floats for
each component).

Test: cts-tradefed run singleCommand cts-dev --module CtsUtilTestCases --test android.util.cts.HalfTest
Change-Id: I7e071edd2b66fdf6b375ce0e3e1a72ec3fb635b5