Migrate resuming/pausing activity to TaskFragment

The resumed, pausing and last-paused activities are now set
at the leaf task fragment.

Also fix some unit tests that having a task containing both
child activity and child task.

Bug: 189384393
Test: build and boot
Change-Id: I0e4b00130ffa624e7dab6d53cbd9f8f95bc5b3d8