Add overriding method of a default method to signature files

Currently, the signature file elides overrides of a default method,
as they are not essential for the compilation of the stubs.
However, in some scenarios, the overrides of a default method may be
essential for the compilation of the reverse dependency of the stubs.
In the scenario seen below:
```
// API
interface ApiInterface {
    public default void bar() {};
}
class ApiClass implements AnotherParentInterface{
    public void bar() {}
}

// App depending on the API
interface AppInterface {
    public void bar();
}
public class AppClass extends ApiClass implements AppInterface {
}
```
Removal of `ApiClass.bar()` will lead to not overriding an abstract method
`AppInterface.bar()` compilation error in `AppClass`.
Therefore, this change adds the overrides of a default method to the
signature files.

Test: checkapi
Bug: 299366704
Change-Id: Ie11dbeafc39efec06f605e306c92155e834a6685