Fix vtable corruption issue

We were adding duplicate methods to the vtable in some cases where
default methods (and conflict methods) were used. This caused issues
where they were not correctly overridden in subclasses that implement
these methods directly. When overridden only one of the vtable entries
was updated meaning it was still possible to reach the overridden code
using a virtual call.

This change prevents the duplicate methods from being added to the
vtable in this circumstance. It also adds a debug check that ensures
that the vtable has no duplicates to prevent regressions.

Bug: 31280371

Test: mma test-art-host
Test: mma test-art-host-run-test-960-default-smali

Change-Id: I17d88fb8949c8d5d75b4de3c734fd98660b81e61