Allow user to select second split app from Taskbar

This patch makes it so that (when we enable Taskbar in Overview) users will be able to select their second app for splitscreen by tapping the Taskbar icon, or the icon in AllApps.

This was done by performing a check to SplitSelectStateController when a taskbar icon is hit. If we are currently in split select mode, Taskbar/AllApps icons will no longer launch their respective fullscreen apps, but instead confirm the split attempt. The confirmed app will either be an already-running instance of the app, or a fresh instance of the app (if none is currently running). The split confirmation function is located in TaskbarUIController, where it is accessible to both LauncherTaskbarUIController (for 1P Launcher) and FallbackTaskbarUIController (for 3P launchers).

Also cleans up ~2 lines of unused code from the old splitscreen instructions toast.

Outstanding issues:
- When selecting a second app from within AllApps, the AllApps shade does not animate away in a satisfying way
- When selecting a second app and launching a fresh instance of that app, the animation appears to come from the wrong location
- Intent + Intent splitting does not currently work
- If the selected app is already running with multiple instances, it picks the oldest instance. Ideally, the newest instance is preferred.

Bug: 251747761
Test: Manual testing with Taskbar in Overview flag enabled
Change-Id: I302dc092740bb880f9134ba8e2e587c4f2c29d01