MathUtils: Remove static Random field.

This is unsafe because the random will be seeded in the zygote, which
means all processes on the system generate the same sequence of random
numbers. Fortunately, this code is only used in tests, so get rid of it.

Note that the Random that backs Math.random() is reseeded after every
fork to avoid a similar issue.

Bug: 35918685
Test: HwAccelerationTest
Change-Id: Ice79aa25bb9017f7a0b91659afe04112850cb74b