Simple implementation of Taskbar Overflow.

- Adds placeholder view to represent the Overflow affordance.
- Overflow button will always appear as long as there are any
  recent apps.*
- Click / long click on the button will open KeyboardQuickSwitch
- Relevant code is Guarded by Flags.taskbarOverflow.

*Note that this is not up to spec. This change is only meant to
setup the codebase to support Taskbar Overflow.

Bug: 368119679
Test: open app, enter desktop windowing, click overflow button
Flag: com.android.launcher3.taskbar_overflow
Change-Id: I47f96797408fda09e8326ba8fb7981c91de80e36