Set random seeds post fork.

Gets rid of static Random objects from various classes and
makes them use Math.random instead. Also add hidden APIs to
set the random seed post fork (where we're single threaded).

This has the nice side effect of fixing a performance bug
related to linpack (11388705) where threads were serializing
on Math.class while calling Math.random.

bug: https://code.google.com/p/android/issues/detail?id=79143
bug: 11388705
Change-Id: Ide3a7d17fe855a8086601348be87a890e4c42ab4