Avoid extra calls into updateOomAdj

finishReceiver involves multiple calls into updateOomAdj
and the last one is from the trimApplications which will do
a full oom adj update; therefore the other one could be skipped.

Also fixed a test failure int ActivityManagerPerfTests due to
background restrictions

Bug: 160908195
Test: atest ActivityManagerPerfTests
Test: atest CtsAppTestCases
Change-Id: I6a7d0a96a65cea24995337eca1b50f00dbc4edee