Handle error if IabActivity is destroyed during purchase flow.

The IabActivity may be destroyed by some abnormal operations.
(For example, pressing home button then clicking app icon to re-enter it.)
The operations will cause Android clean the activity stack if the application lauchMode is 'SingleTask'.

If the activity stack is cleared during purchase flow, we shall handle it and fire an unexpected error.
In this situation, the activity's onDestroy() function will invoked before onActivityResult().
So we shall implement onDestroy() for IabActivity.