Define exportable full api surface stubs

The two major roles of the full api surface stubs (i.e.
android_*_stubs_current) are:
- Appended to classpath when a module sets sdk_version
- Input to generating hiddenapi flags and determine the accessibility of
  the api at runtime

With Trunk Stable, all flagged apis that are not enabled should not be
accessible at runtime. In other words, the currently existing full api
surface stubs that include both non flagged and flagged apis should not
be used to generate the hiddenapi flags.

To resolve this problem, this change introduces full api surface stubs
of the "exportable" stubs (i.e. non flagged apis + flagged apis enabled
by release configuration).

The new modules are named similarly to the currently existing full api
surface stubs, but appends the "_exportable" prefix to differentiate
with the currently existing modules. These modules are solely used for
hiddenapi flags generation, and the currently existing modules continue
to serve the first aforementioned role.

The new modules will toggle between the from-text stubs module and the
from-source stubs module when the exportable stubs can be generated from
the from-text stubs generating `java_api_library` module. For now, these
modules are always built from source.

Test: m android_stubs_current_exportable
Bug: 317426356
Change-Id: I1f6f738ae9c5ebf151a1554aa1e357ef0092c89c