Use Root in OpenActivityFromBubbleOnLocksreenTest

This test is currently broken because we're trying to find floating
views when we're showing bar views. Switch to Root which supports
both bubble modes.

Bug: 433351180
Flag: EXEMPT test only
Test: OpenActivityFromBubbleOnLocksreenTest
Change-Id: Ief3df1e16eb28eae9fe945f82a6243945791b1b9