Fix juttering problem with split staging animation

This patch makes it so that app icons no longer jutter uncomfortably when staging and confirming a split from home.

The problem occurred because I used Math.round when calculating the position at which to center the app icon within the SplitPlaceholderView. When an inexact integer is used to calculate the x and y position on every frame, it creates juttering, especially when numerical values are small.

Solved by removing Math.round and using floats going forward.

Fixes: 262308025
Test: Manual
Change-Id: If91d1d3ee52652c155ed7e4e1d4f4620fdc41cc9