Stubs variant is used when building for APEX

When a native module is built for an APEX and is depending on a native
library having stubs (i.e. stubs.versions property is set), the stubs
variant is used unless the dependent lib is directly included in the
same APEX with the depending module.

Example:

apex {
    name: "myapex",
    native_shared_libs: ["libX", "libY"],
}

cc_library {
    name: "libX",
    shared_libs: ["libY", "libZ"],
}

cc_library {
    name: "libY",
    stubs: { versions: ["1", "2"], },
}

cc_library {
    name: "libZ",
    stubs: { versions: ["1", "2"], },
}

In this case, libX is linking to the impl variant of libY (that provides
private APIs) while libY is linking to the version 2 stubs of libZ. This is
because libY is directly included in the same apex via
native_shared_libs property, but libZ isn't.

Bug: 112672359
Test: apex_test added
Change-Id: If9871b70dc74a06bd828dd4cd1aeebd2e68b837c