Fix bubbles being on the wrong side after fold -> unfold

When bubbles are on the right hand edge of the device before unfolding
the position after unfolding would be on the left instead of the
right.

This was happening because the resting stack position is based in
the previous bounds, not the new ones.

To fix this, when the bounds change in BubblePositioner, the resting
stack position can be saved as a relative stack position using the
previous bounds and then restored using the new bounds. This converts
it to the correct spot on screen.

Test: manual - have bubbles on right side of fold phone, unfold
             => observe they are still on the right
             - check bubble position after orientation changes
Test: atest BubblePositionerTest
Bug: 256993976
Flag NONE; bug fix

Change-Id: Id515efa1c2aa32942e6d2eaffe69aa69f40bf5ca