Handle missing ActivityTaskManager service

ActivityTaskManager.getService can return null if
SystemManager.getService(Context.ACTIVITY_TASK_SERVICE) returns null.
StrictMode.registerBackgroundActivityStartCallback needs to check for
this so it doesn't try to register a callback on a non-object.

Bug: 376302055
Bug: 324089586
Test: atest CtsPrintTestCases
Change-Id: If71e95981f1c7c23e21968405ae3e30149d16417