DO NOT MERGE: ActivityManager#killBackgroundProcesses can kill caller's own app only

unless it's a system app.

Bug: 239423414
Bug: 223376078
Test: atest CtsAppTestCases:ActivityManagerTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:fa94ce5c7738e449cb6bd68c77af4858018e49e0)
Merged-In: Iac6baa889965b8ffecd9a43179a4c96632ad1d02
Change-Id: Iac6baa889965b8ffecd9a43179a4c96632ad1d02