Request split launches using a new enum, snapPosition, rather than a float splitRatio

Split ratios are now stored as an int enum rather than float values. Previously, split ratios were stored as floats, which needed to be recalculated frequently and differed depending on device screen size. Now Shell to store and launch split screen setups using an enum rather than floats.

Bug: 274835596
Bug: 182839788
Test: Existing tests still pass
Change-Id: Iebb8d3c7e8f9d597a59b677e48072300ddea458f