Remove usage of Math.randomLongInternal

As a part of internal libcore API cleanup some of the functions
previously exposed are getting removed from public surface.
Math#randomLongInternal is a wrapper around java.util.Random and has no
specific implications so its usages are get refactored.

Bug: 154796679
Test: m droid
Change-Id: I29e0e9307fbaf9c1ac018b83014efb2d3dd74479