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
Merged-In: I35d20539ffac055a6d61260445620f45584bd9c5
Merged-In: Ieed6af77da1bc31cfecc5272b9f97971db7ae7b2
Merged-In: I8b8a427ee87339cc038e53adc0912283b05d2cfc
Change-Id: Iac6baa889965b8ffecd9a43179a4c96632ad1d02