setDefaultImpl aborts on a second call

The actual problem is that default implementation is set globally.
setDefaultImpl might not work as expected when it is called twice with
different instances.

Because we don't have a proper solution for the problem, we prevent
calling setDefaultImpl() twice by aborting.

Bug: 140139809
Test: ./runtests.sh (in /system/tools/aidl)
Change-Id: I659d3eaad3a45dcba608fa79a08f083f84bc4d58