Add TV splitscreen menu

Add a splitscreen menu for TV allowing to change the focused task, swap
the stages and exit the splitscreen mode by closing one of the stages.

Bug: 242179031
Test: Enter the splitscreen mode. Send the corresponding broadcast to open the menu (adb shell am
   broadcast -a com.android.wm.shell.splitscreen.SHOW_MENU). Choose the needed action button and press it, open the menu again, etc. Press the back button on the
menu control to hide the menu window.

Change-Id: Iac17445414a190c91ccd407d67f0f8c29d6bd4bf
Merged-In: Iac17445414a190c91ccd407d67f0f8c29d6bd4bf
(cherry picked from commit 7ab2a40d)