Add error logging and better crash safety for app pairs

This causes app pairs to log an error message if creation or icon inflation is attempted with contents != 2. App pairs with wrong numbers of members should not be created anymore.

Fixes: 317283244
Fixes: 322892793
Test: Manual
Flag: ACONFIG com.android.wm.shell.enable_app_pairs TEAMFOOD
Change-Id: I5e454042719bb67187f900c546380bb44cfc34ac