Add AMS lock protection to mAppsInBackgroundRestricted collection

When the process is started asynchronously, the mAppsInBackgroundRestricted collection lacks AMS lock
protection and will throw ConcurrentModificationException
crash with a low probability.

Bug: 236582924
Test: Treehugger
Change-Id: Ieec1de9ee1fd336cf1de54566f80442e3cd3acdd

services/core/java/com/android/server/am/ProcessList.java

@@ -2269,7 +2269,9 @@ public final class ProcessList {
                 final boolean inBgRestricted = ast.isAppBackgroundRestricted(
                         app.info.uid, app.info.packageName);
                 if (inBgRestricted) {
-                    mAppsInBackgroundRestricted.add(app);
+                    synchronized (mService) {
+                        mAppsInBackgroundRestricted.add(app);
+                    }
                 }
                 app.mState.setBackgroundRestricted(inBgRestricted);
             }