DO NOT MERGE: Fix ActivityManager#killBackgroundProcesses permissions

In the pevious CL, we incorrectly added the permission check in the
killBackgroundProcessesExcept. Now fix this issue.

Bug: 239423414
Bug: 223376078
Test: atest CtsAppTestCases:ActivityManagerTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:9a14267036edef6224a7418f92b9d3f88f2ac5b7)
Merged-In: I9471a77188ee63ec32cd0c81569193e4ccad885b
Change-Id: I9471a77188ee63ec32cd0c81569193e4ccad885b