Rename DragResizeCallback to DragPositioningCallback.

This CL renames DragResizeCallback to DragPositioningCallback to better
reflect the interface's purpose of handling both drag to move and drag
to resize events.

Bug: None
Test: Manual: drag windows to resize and move
Test: atest WMShellUnitTests

Change-Id: I89eb1413a8a0c69eb1e75829ca65711a114e7a18
8 files changed