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:140fce861944419a375c669010c6c47cd7ff5b37)
Merged-In: I9471a77188ee63ec32cd0c81569193e4ccad885b
AOSP-Change-Id: I9471a77188ee63ec32cd0c81569193e4ccad885b

Change-Id: I1b1e683b6a92b0fa2a844a99bedcccac8c980e58