Taskbar overflow: only show if apps don't fit + limit # of icons

- Only shows the overflow icon if all of the recent apps do
  not fit in the taskbar.
- Limits the number of recent app icons on screen so that they
  do not go out of bounds.
- Removes the overflow if it's not needed.

Bug: 368119679
Test: open app, enter desktop windowing, overflow only shows
      when reaching max number of apps
Flag: com.android.launcher3.taskbar_overflow
Change-Id: I96040495ee38f67bce8e41f9eb4ec17fcef5189a