Incremental repairs to side by side stacks.

- Add taskId parameter to createStack() so stacks are pre-populated
with a task.
- Keep track of stack access order in DisplayContent so getTasks
returns in MRU order.
- Set touchableRegion in InputMonitor so modal touching does not
extend beyond stack boundary.
- Fix stack merging so that deleting a stack results in a new
stack the size of the two children.

Change-Id: I62a6ba0a34f34dd7ec866b440bf04595379e19e8