Use module name as the suffix for apex variant

apex {
    name: "myapex",
    native_shared_libs: ["libfoo"],
    apex_name: "apex_name",
}

override_apex {
    name: "myapex.override",
    base: "myapex"
}

Previsouly, above wasn't supported because both APEXes have the same
apex_name and that apex_name is used as the suffix of libfoo. i.e.,
there are two libfoo.apex_name modules defined.

Now, the two apex variants of libfoo are named as
libfoo.myapex and libfoo.myapex.override.

Bug: 140136207
Test: m
Change-Id: I63f8a1de463011c6e0b97f5f6eee83103e22bc30