APEX can depend on non-APEX module and vice versa

Previously, an APEX module (also called apexBundle inside of Soong) had
only one "apex" variant which has the same name as the module. This
prevented an APEX from depending on another module which is outside of
the APEX (ex: another APEX). Similarily, a module outside of an APEX
(ex: a shared lib or a test) couldn't depend on an APEX.

This CL fixes the issue by creating the "" variant as the alias of the
"<apex_name>" variant, and also by setting the "" variant as the default
dependency variant.

Bug: 321626681
Test: m
Change-Id: Ie3e57a96530c25e966cfd551676d819c442bb0d5