Addition separation of adding vs. reparenting activity

- Removed method ActivityRecord.setTask which did only half of what
is needed when adding a task and also only did half of what is needed
for reparenting. Previous callers now need to call either
ActivityRecord.reparent() to reparent and activity or
TaskRecord.addActivityAtIndex() to add an activity to a task.
- Fixed some NPE that resulted from the above change.

Test: adb shell am instrument -w -e class
com.google.android.setupwizard.tests.activity.SetupWizardExitActivityTest
com.google.android.setupwizard.tests.activity
Bug: 34179495

Change-Id: Ic69487f51d1bd139825e6a8054a49ce143065a57