Refined comment in FP abs() implementation.

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 (signaling NaN may become quiet though).

Test: test-art-host
Bug: 30758343
Change-Id: I8d9ca9aa58a0ffbde6120370f7aecd02dc7395e0