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:d1c95670b248df945784b0f2830acf83b5682de3)
Merged-In: Iac6baa889965b8ffecd9a43179a4c96632ad1d02
AOSP-Change-Id: Iac6baa889965b8ffecd9a43179a4c96632ad1d02

Change-Id: I3a39b5e2b2ff0c314972ddeccb012894de704de8