Add logging to DragToDesktop mergeAnimation

To avoid getting stuck during the DragToDesktop transition we might need
to finish the transition early if we receive merge-requests from other
transitions.
With this CL we add logs to see what transitions are requesting merges
into DragToDesktop.

Bug: 397138738
Flag: NONE adding logging
Test: N/A
Change-Id: I9e4e1fea2b1dff5af0fcdd0ee1c3a710a654858e