Ignore taskbar launch request if app(s) are already running

This CL fixes several issues:
- When launching an app pair on top of the exact same app pair, Overview no longer enters a broken state
- When launching another app quickly after launching an app pair, splitscreen no longer breaks

Fixes: 315190686
Test: Launch app pair and then launch same app pair again from Taskbar -- Overview does not break
Flag: ACONFIG com.android.wm.shell.enable_app_pairs DEVELOPMENT
Change-Id: I0d474dd89359e1a10a881c5cf8deee64754219c3