Prevent ViewRootImpl.mDisplay become null.

For some timing cases client side may not get desired display with old
displayId.
For this issue, client side has just received configuration change and
handle it in ActivityThread#handleActivityConfigurationChanged. At the
same time, DisplayManagerService was about to destory display. When
ViewRootImpl want to update display with new DisplayAdjustments, it got
null because the old displayId was removed.
For this scenario, fallback to use default display. Usually there should
be other displayId change event comes from server side to ask client update.

Change-Id: Ifb843f82b753761cb6306e7482b38a0ffd89498d
Fix: 73558361
Test: atest CtsWindowManagerDeviceTestCases
Test: atest CtsActivityManagerDeviceTestCases