Improve null safety for app pairs

- Improves crash safety in AppPairIcon.java. An app pair should never have more or fewer than 2 app members, but if it happens somehow and there is an app pair on the home screen, the device will enter a crash loop that is hard to recover from. Added some safety checks so that it doesn't happen.

Fixes: 317283244
Test: Manual and presubmit tested
Flag: ACONFIG com.android.wm.shell.enable_app_pairs TEAMFOOD
Change-Id: I2cbd90f028f7c31af6e832b5515502ba992f0b54