Folder support in Taskbar

- Add TaskbarActivityContext which allows shared Launcher elements to
  "just work" using existing generic ActivityContext.
- TaskbarContainerView extends BaseDragLayer<TaskbarActivityContext>.
- Inflate FolderIcon and Folder using TaskbarActivityContext to be
  shown in TaskbarContainerView.
- Use TaskbarActivityContext's DeviceProfile to determine icon size
  instead of overriding in styles. This also ensures that normal
  BubbleTextView icons have the same size as FolderIcons.

Test: Place a folder in home screen hotseat, ensure it shows up in
taskbar and can be opened, and that apps inside it can be launched
or dragged.
Bug: 171917176

Change-Id: Ic25d2f84bcd7e3399c88989305ea565497c030d9

src/com/android/launcher3/DeviceProfile.java

@@ -375,7 +375,7 @@ public class DeviceProfile {
      * iconTextSizePx, iconDrawablePaddingPx, cellWidth/Height, allApps* variants,
      * hotseat sizes, workspaceSpringLoadedShrinkFactor, folderIconSizePx, and folderIconOffsetYPx.
      */
-    private void updateIconSize(float scale, Resources res) {
+    public void updateIconSize(float scale, Resources res) {
         // Workspace
         final boolean isVerticalLayout = isVerticalBarLayout();
         float invIconSizeDp = isVerticalLayout ? inv.landscapeIconSize : inv.iconSize;