Move Half implementations to libcore to allow ART optimizations

Move Half FP16 implementations to libcore, to allow ART compiler to
optimize these methods with intrinsic implementations.

For example, on ARM64 with ARMv8.2 FP16 half registers and instructions:
- Half toFloat/toHalf can be implemented with FCVT;
- Half floor/ceil/round can be implmented with FRINT(pna);
- Half max/min can be implmented with FMIN/FMAX.

Such fast Half FP16 intrinsics can help accelerate ColorLong ARGB
encoding/decoding in Android framework.

Change-Id: I6225ebf8aa825b0394ce8f13e12db317f5c6e3fd