Fix an issue where the start position of the stack is wrong in RTL

In RTL, when the stack shown for the first time with user education,
the stack would be positioned offscreen. This is because we were
incorrectly passing the whole screen size in getDefaultStartPosition()

This would work fine when the stack starts on the left, but not in
RTL when it starts on the right and the size of the stack needs to be
subtracted.

To fix this I moved over getAllowableStackPositionRegion into
BubblePositioner and use that when retrieving the starting position.

Test: manual - be in RTL, show user education, observe that the
      stack is fully on screen as expected. Also check in LTR.
Bug: 233402922
Change-Id: Ib8ad2a289f373fdc1c61527c47793461fc34d929