Reduce object allocation in AMS.computeOomAdjLocked()

Follow-up clean-up CL to ag/4246927 where we introduced structured
way of communicating process state to WM. However, in that CL we
were creating a few objects whenever AMS.computeOomAdjLocked() is
called. This CL corrects that with a temp. object/class pattern.

Test: existing tests pass.
Bug: 80414790
Change-Id: I97241b3d71e10610cca562397d33c8fa77727efa