Don't remove the overflow from the bubble bar

When a new bubble is added to the bar and we're over the limit
remove the second to last bubble instead of the overflow.

This change also updates the key of the overflow bubble to
match the one in wmshell.

Bug: 287121592
Test: manual - Add 6 bubbles to the bubble bar
             - The overflow bubble should still be there
Change-Id: Ibd3ab73be5d3afb49f771a1925ebda2c45b55136