Unity 1.7

fix(billing): prevent NPE when iabConnection becomes null during setup

A race condition allowed iabConnection to become null between the
service connection callback and the billing support check, causing:

java.lang.NullPointerException: isBillingSupported(...) on null

We now:

• retry once via broadcast when the service connection is lost
• fail gracefully if the broadcast connection is also lost
• avoid infinite fallback loops

This makes the setup flow null-safe and crash-free.