Allow the dismiss target size to be overridden

For floating tasks the normal target size is too small to really
fit the task window, this allows the dismiss target to have a
different size set via a resource id.

Test: manual - override the size (in different CL), observe that
               the target is at that size.
Bug: 237678727
Change-Id: I85edae739992662852d4cf40c250714a100a67bb