add apex_available_name property to apexBundle

This property allows developers to create multiple modules that provide
the same APEX implemenation with some differences. For example, an APEX
that targets the current SDK can be developed alongside an APEX that
targets a pre-S SDK. Common dependencies of these APEX modules can both
use the value of the apex_available_name property in their
apex_available lists and be packaged in either APEX module.

Bug: 285138555
Change-Id: Ia2cb027e2f19bd6d3d1c02e3d3742a0122a43c97