OomAdjust Performance Improvement

* Reduce the number of processes to be computed.
* Reduce the overhead of getting process attributes

Overall by average the time spent on the OomAdjust update goes
down from 911us to 441us (51.6% faster) in Day-Of-Use test.

Bug: 135941673
Test: forrest test health/reliability/greenday
Test: Loop test
Test: atest MockingOomAdjusterTests
Test: atest FrameworksServicesTests:ActivityManagerServiceTest
Test: atest CtsAppTestCases:ActivityManagerProcessStateTest

Change-Id: I43cc4af4a268b24620c3231a87f7c9d2a14e4fbc