Update how PIP size is determined

- minSize = 108dp
- defaultSmallestEdge = max(23% of screen width, minSize)
- the shortest edge of the PIP should be minSize and the rest scales
  according to the aspect ratio
- rather than a default PIP size, use default aspect ratio
- adding expand button
- fitting actions to spec

Fixes: 35358504
Test: manually used test app to try different aspect ratios
Change-Id: Ib6890fb7824889b9edeea7efb5b9771e64fc1514
Signed-off-by: Winson Chung <winsonc@google.com>