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
ddc1cb2c15