Bottom align bubbles on large screen portrait

This CL bottom aligns the bubbles on large screen portrait. When the
IME appears, the bubbles currently get clipped / inset. Looking into
shifting them up / animating them instead of insetting them in a
separate CL.

Adds BubblePositionerTests for the new alignment.

Test: manual - expand a max height bubble & small height bubble on
               large screen in portrait
             => observe that they are bottom aligned
             - rotate the device
             => observe that they are vertically centered
             - verify that phone still works as normal in portrait
               and landscape
Test: atest BubblePositionerTest
Bug: 310670691
Change-Id: Ic965e183e9cdb852b0fa90b5adad8d7f3097f18f