Added support for static vs. dynamic stacks

Now that stacks represent workspaces we can define static
stacks which help shape the characteristics of the tasks
they contain. For example, fullscreen tasks/activities will
normally be launched in the stack with id
FULLSCREEN_WORKSPACE_STACK_ID, while freeform tasks/activities
will normally be launched in the stack with id
FREEFORM_WORKSPACE_STACK_ID.

Also, added ability to position a task at any index in a stack.

Bug: 22068114
Change-Id: Ib6c62a84b5f204fbf072755264c5c5eda6184f97