Create APEX variant for prebuilt_apex/apex_set

Previously, the prebuilt_apex/apex_set did not have an apex specific
variant created which meant that they depended upon the platform
variant of the modules it depended upon. This change creates an
apex variant for them just as is done for the apex module type which
causes it to depend upon the apex specific variant of the modules it
depends upon.

Test: m droid
Bug: 179354495
Change-Id: I7d6f3609c267b3e90b90b9befe7d76f351a0c2bd