Fix java.util.Random performance regression.

With the monitor change in ART, getting the identity hash code of an
object puts the hash code in the lock word. This hurts performance
if you synchronize on said object since it makes it that we always
go slow path for monitor enter and exit. Although the fix
allocates a new object in the Random constructor, it should not cause
performance problems assuming people don't excessively create new
random number generators.

Change-Id: I750e57fb7c5f70e66ad85d19e2c065e9e52a613f