Un-hide FloatProperty and IntProperty

These simple utility classes (used internally since Property was first introduced
way back in Jellybean) are the best way to avoid autoboxing when using the Property
approach to ObjectAnimator. But since their hidden, developers have to use the
autoboxing version (Property, the superclass) or know to come up with their own
primitive-optimized property subclasses.

This CL simply un-hides these two classes to make them usable outside of the framework.

Issue #21722783 Make [Int|Float]Property public

Change-Id: I7f3a456d108bf48587d711255d3577cce3ac5e24