Flexible maximum number of bubbles to show

* adjust the default spacing so that its small enough that 5 bubbles
  can show on crosshatch (5 is the default)
* Add max bubbles to BubblePositioner, replace the other usages

Test: atest BubbleDataTest
Test: manual - have 5 bubbles
             - adjust screen size to largest
             - open the stack
             => notice that the number of bubbles have changed and
               they all fit on the screen without overlap
             - repeat & check the different densities
Bug: 187715444
Change-Id: I4794ed89541eeb573c367748f980cf7ffe45d3f3