RESTRICT AUTOMERGE Create separated tasks for different apps from startActivities

Assume there are 2 applications A, B with different uids.
There are 4 activities A1, A2, B1, B2 with default task
affinity and launch mode.

After A1 called startActivities(B1, A2, B2):
 Original   : Task(A1, B1, A2, B2)
 This Change: Task(A1, B1), Task(A2, B2)
In other words, the source caller cannot launch its activity
above the activity of other application in the same task, and
it can still launch activity of other application in its task.

Bug: 145669109
Test: run cts --test android.server.cts.StartActivityTests \
      -m CtsServicesHostTestCases
Change-Id: I97bd875146a52f62b8fe82235487ccefb2955e8e

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

@@ -951,6 +951,8 @@ class ActivityStarter {
         } else {
             callingPid = callingUid = -1;
         }
+        boolean forceNewTask = false;
+        final int filterCallingUid = callingUid >= 0 ? callingUid : realCallingUid;
         final long origId = Binder.clearCallingIdentity();
         try {
             synchronized (mService) {
@@ -970,6 +972,9 @@ class ActivityStarter {
 
                     // Don't modify the client's object!
                     intent = new Intent(intent);
+                    if (forceNewTask) {
+                        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
+                    }
 
                     // Collect information about the target of the Intent.
                     ActivityInfo aInfo = mSupervisor.resolveActivity(intent, resolvedTypes[i], 0,
@@ -995,7 +1000,17 @@ class ActivityStarter {
                         return res;
                     }
 
-                    resultTo = outActivity[0] != null ? outActivity[0].appToken : null;
+                    final ActivityRecord started = outActivity[0];
+                    if (started != null && started.getUid() == filterCallingUid) {
+                        // Only the started activity which has the same uid as the source caller can
+                        // be the caller of next activity.
+                        resultTo = started.appToken;
+                        forceNewTask = false;
+                    } else {
+                        // Different apps not adjacent to the caller are forced to be new task.
+                        resultTo = null;
+                        forceNewTask = true;
+                    }
                 }
             }
         } finally {