Fix vtable corruption.

Due to failing to keep track of superclass implementations of
interface methods we could end up in situations where methods were
placed onto a class's vtable multiple times. This could cause virtual
and interface dispatches on subclasses to fail by causing corruption
of the subclass's vtable and iftable.

Bug: 28333278

Change-Id: I37d9740ca912daf37cdf9ff82697bbc5db46177a