Move implementation of fract(float) into driver/runtime.

Move the implementation of the float variant of fract from rs_math.rsh
into driver/runtime. The old implementation remains in place for older
versions, and the move occurs only for API level >= 24.

Signed-off-by: Verena Beckham <verena@codeplay.com>
Change-Id: Id94e7191604d5ccaee41b6634731ee997bf01983
diff --git a/api/rs_math.spec b/api/rs_math.spec
index b390610..f686bab 100644
--- a/api/rs_math.spec
+++ b/api/rs_math.spec
@@ -1079,7 +1079,7 @@
 end:
 
 function: fract
-version: 9
+version: 9 23
 attrib: const
 w: 1, 2, 3, 4
 t: f32
@@ -1090,6 +1090,14 @@
  return fract(v, &unused);
 end:
 
+function: fract
+version: 24
+w: 1, 2, 3, 4
+t: f32
+ret: #2#1
+arg: #2#1 v
+end:
+
 # TODO Add f16 frexp
 function: frexp
 version: 9