Show last selected bubble when stack expands

Fixes: 152888262

Test: manual
        1. Have 5 bubbles, select -middle- bubble, select overflow
        2. Collapse stack, reopen stack
        -> See that middle bubble is now first in stack and selected

Test: manual
        1. Have 5 bubbles, select -first- bubble, select overflow
        2. Collapse stack, reopen stack
        -> See that first bubble is selected again

Test: manual (overflow disabled)
        -> Previously selected bubble shows first in stack on expand

Change-Id: I8749306be247be3136d2b8a36852e796a5195ef7