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