[HostStubGen] Support non-static methods...

... with native substition classes.

It turned out native methods can be non-static. Now we handle them in this way:
- The substitution method still needs to be static.
- And take the original 'this' as the first argument.

The first argument type must match the source class.
(e.g. the arg type can't be `Object`)

Test: run-all-tests.sh
Bug: 292141694
Change-Id: I9a53e1c155e138e466589c1a7684dcc86601e9c8