Fix the IntentTests test case failed

NPE occurs when process the pending reparent activities on
ResetTargetTaskHelper. Activities that enable the
allowTaskReparenting were not reparented to other tasks
while an activity was started with RESET_TASK_IF_NEEDED
flag.

This CL refactor the reparent path to fix the NPE.

Bug: 150303302
Test: atest CtsWindowManagerDeviceTestCases:IntentTests
Change-Id: I55f4eed0e6be8a2f59757fe9308b726196a1f011