Optimize Process thread priority method calls

Annotate native calls with @FastNative and @CriticalNative for several
priority-related Process methods. Also ensure the the
Process.setThreadPriority overloads are compiled in the boot image, as
these were only recently made Java methods (versus native trampolines).

Bug: 417492941
Bug: 441066069
Bug: 374158990
Flag: EXEMPT BUGFIX
Test: presubmit
Change-Id: Id6074389ef4dedf13f25119fc35e5f9f968476f1