Fix the reset of boosted zygote thread priority.

This fixes the unexpected priority 112 of the daemon threads
(eg. HeapTaskDaemon). The problem was that when the zygote main
thread's priority is reset, it directly calls setpriority() and fails
to update the priority value in java.lang.Thread, which in turn causes
any threads created by the thread to unexpectedly inherit the boosted
priority. Calling java.lang.Thread.setPriority instead fixes.

Bug: 35801778
Bug: 28866384
Test: angler master userdebug boots and thread priorities checked.

Change-Id: I68a6ed7244a9067acc2749feca7f88422bf44b02