Fix app pairs launch from in-app Taskbar

This CL adds a handler function in SplitAnimationController that manages the complicated logic flow for launching an app pair when already inside of an app.

To give an idea of the complicated logic:
If the user tapped on an app pair while already in an app pair, there are 4 general cases:
  a) Clicked app pair A|B, but both apps are already running on screen
  b) App A is already on-screen, but App B isn't
  c) App B is on-screen, but App A isn't
  d) Neither is on-screen

If the user tapped an app pair while inside a single app, there are 3 cases:
   a) The on-screen app is App A of the app pair
   b) The on-screen app is App B of the app pair
   c) It is neither

For each case, we call a different animation and launch the app pair in a different way.

When merged, this patch will fix all animation glitches that are currently happening in these situations, and get us 90% of the way to having the ideal animation in all cases. There are still a few complicated cases that need a polished animation (like when you launch app pairs with custom ratios), which will be implemented in a following patch soon (I thought this CL was big enough already as is).

Bug: 316485863
Fixes: 315190686
Test: Manual testing of all the different launch combinations
Flag: ACONFIG com.android.wm.shell.enable_app_pairs TEAMFOOD
Change-Id: I5c0e03512bb706360c575d833cac6ed02a5de936