Don't mock out TaskRecord & ActivityStack during test (56/n)

Use TaskBuilder and StackBuilder to create objects needed for tests so
we are not mocking out TaskRecord and ActivityStack when we create Task
and TaskStack for tests.

Bug: 80414790
Test: Existing tests pass
Change-Id: I537d765576859ecfdd0152e95e4d9261d74fd624
6 files changed