Adding MSDL feedback to dragging apps and widgest over drop targets.

The ButtonDropTarget class delivers MSDL feedback onDragEnter.

Test: DeleteDropTargetTest
Test: manual. Verified haptics when dragging apps and widgtes over
  "Remove" and secondary drop areas.
Flag: com.android.launcher3.msdl_feedback
Bug: 371249457
Bug: 371315650
Change-Id: Ic3f79ab5e3da7bb26797f2a1bfb79b5e37b3ac97