Fix NPE when referencing DragResizeInputListener

Some window decorations may have a null DragResizeInputListener (for
example, if the task is unresizable). Check for null before calling
isHandlingDragResize() when handling motion events.

Bug: 322571436
Test: drag-resize an unresizable freeform app, verify no NPE.
Change-Id: Idcd176df9fd9eb974635e87d89befbd4ef62ff44
2 files changed