Invalidate recent tasks when the splits (or split info) changes

- Now that split bounds info is tied to the recent tasks provided to
  Launcher, we need to notify of the changes when the info changes

Bug: 206868465
Test: atest WMShellUnitTests
Test: Split app pair, go to overview, go back to app, change divider
      position, then go back to overview -> ensure pair is drawn
      correctly
Change-Id: Id7926a20ab3be3a9cb5eb832fb321c80bd60bae0