Revert^2 "Always require opt-in for root/system"

This reverts commit 35cbe4dc.

Reason for revert: rolling forward - rollback did not fix flakiness issue

Change-Id: Iebaf786ee15954da1f8dd4fe64cc610fb76b4357

services/core/java/com/android/server/wm/BackgroundActivityStartController.java

@@ -28,6 +28,7 @@ import static android.content.pm.PackageManager.PERMISSION_GRANTED;
 import static android.os.Build.VERSION_CODES.UPSIDE_DOWN_CAKE;
 import static android.os.Process.INVALID_PID;
 import static android.os.Process.INVALID_UID;
+import static android.os.Process.ROOT_UID;
 import static android.os.Process.SYSTEM_UID;
 import static android.provider.DeviceConfig.NAMESPACE_WINDOW_MANAGER;
 
@@ -385,6 +386,10 @@ public class BackgroundActivityStartController {
                     return BackgroundStartPrivileges.NONE;
                 case MODE_BACKGROUND_ACTIVITY_START_SYSTEM_DEFINED:
                     // no explicit choice by the app - let us decide what to do
+                    if (callingUid == ROOT_UID || callingUid == SYSTEM_UID) {
+                        // root and system must always opt in explicitly
+                        return BackgroundStartPrivileges.NONE;
+                    }
                     if (callingPackage != null) {
                         // determine based on the calling/creating package
                         boolean changeEnabled = CompatChanges.isChangeEnabled(