binderSafeInterfaceTest: another quick flake fix

P.S. we should delete safe interface and move everything
to using AIDL-generated interfaces. It doesn't support
all the things AIDL does, and we don't want so many
ways of doing things.

Change-Id: I92c97e458314c0c85ec9ca8eab2c1552abec8ca6
Fixes: 387346410
Test: TH