Cleanup drag and drop flow/code organization

- Pulled out common utilities on ClipData into a separate DragUtils
  class
- Decouple the drag session from the DragAndDropPolicy (it never really
  made sense to have it there since DragLayout uses it as well)
- Remove obsolete/unused drawable
- Separating drag and drop logtag

Bug: 290391688
Test: atest WMShellUnitTests

Change-Id: I25aff7ec7a6b3977a1f528e4736c40da986718e5
Signed-off-by: Winson Chung <winsonc@google.com>