Adjust Java Language thread priority in new processes.

Previously init started the Zygote with -20 nice value and then
immediately switched it to 0 in ZygoteInit.main so that new applications
forked from the Zygote have the correct Java Language thread priority.
This patch delays this priority change so that it only affects the new
processes and not the priority of the Zygote.

Test: m & flash & check ps
Change-Id: Id00d0cfd642f02640c40b6e7b2aa8933a320b60c