Only setup SecondaryDropTarget in onDragStart

- In LauncherAccessibilityDelegate.getsupportedActions, it calls supportsAccessibilityDrop on each button regardless if DropTargetBar is visible or not. SecondaryDropTarget.supportsAccessibilityDrop attempts to setupUi which causes measure/layout of DropTargetBar (as well as DragLayer) each time LauncherAccessibilityDelegate.getsupportedActions is called
- To avoid this, only setupUi in onDragStart. In other cases don't call setUi.
- Also fixed a bug that when accessibility is on, upon dragging a suggested app, the secondary action suddenly changed to Uninstall

Fix: 233383721
Test: Enable accessibility, observe no excessive amount of onLayout is called on DropTargetBar
Change-Id: I4d1a00d320cceed81fa1312b5fc5f1081637bea8