Convert apex_available (for supported modules) to bazel tags.

The tags will then be read by an aspect applied from apex attributes to
validate that the deps are explicitly opt-in to the appropriate apex.

The semantics of this is the same as the Soong apex_available property.

This also adds the "base_apex_name" attr to override_apex conversion so
that apex_available checks continue to work on the non-Google and non-Go
apex names.

Test: presubmits
Fixes: 218841706
Change-Id: I19b3637da9aa47573b252d74f62b601fbdc3784d