core/data_device: update offer position on motion

Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>
core: notify data target when data device leaves

This helps for testing dnd event flow.

Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>
core/test_dnd: test more of the dnd offer states

Add checks for offer positions and offer enter/leave.

Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>