Use TaskFactory to create ActivityStack.

ActivityStack now can be used as a Task by itself, so we need to make
sure they are of right type.

Also changed the signature of that constructor to make it more natural.

Bug: 150409355
Test: WM smoke tests & TH.
Test: atest TaskRecordTests
Change-Id: Ia6aa8decd0af79e58344442e93bd15ca0e373fc9