Changed semantics of FP Math.abs() to impact bit sign (but nothing else).

Rationale:
As a "quality of implementation" rather than pure "spec compliance"
we require that Math.abs() clears the sign bit (but changes nothing
else) for all numbers, including NaN.

**NOTE**
This CL has a corresponding Art test.

Test: test-art-host
Bug: 30758343
Change-Id: I990c5ac72d2a3263b7b1d57dd6da4fabe7f8e274