Update taskbar overflow button

Creates a new view for the taskbar overflow button that draws up to 4
recent item previews. The items are stacked on top of each other in
counter clockwise order, with overlapping bounds, recent items closer
to the top. The item icons have a 2 dip ring around them, of the taskbar
background color.

Adjusts logic to calculate which items become part of the overflow
button, so more recent items get shown in the taskbar.

Initial consideration was to usse FolderIcon to represent the overflow
button, but decided against it because:
*    FolderIcon is fairly entangled with the associated folder view
*    item information uses different data structure (ItemInfo) than
     recent items (GroupTasks)
*    item preview layout within the main icon is similar, but
     sufficiently different that using clipped folder layout rules felt
     like hacking around assumptions made for folder icon UI

Bug: 368119679
Test: Keep opening apps until the task bar enters overflow - verify that
overview button shows up, and contains least recent task
representations. Keep adding items, then closing windows, and verify
the icon gets updated accordingly. Done in landscape and portrait, and
ltr and rtl layout.
Flag: com.android.launcher3.taskbar_overflow

Change-Id: I2824cb0db1f7516ebd74361ce00fb8887857325d