Add nullchecks to ScreenPinningRequest.

To make ScreenPinningRequest workable with devices that does not have
the recent view and navigation bar.

Also mark ScreenPinningNotify methods as public so that it can be
invoked from vendor customized code.

Bug: 78248081
Test: Manually invoke Activity#startLockTask()
Change-Id: I7b3578bfe32dffb32ca1c21f5e30f98aa452efb0