Add aconfig declarations to f/b droidstubs

This change pass the aconfig_declaration modules defined in
AconfigFlags.bp to the *-api-stubs-docs-non-updatable droidstubs module,
so that the "exportable" stubs (i.e. the stubs only containing the
flagged apis that are enabled by the release configuration) can be
generated.

Test: m api-stubs-docs-non-updatable && m checkapi
Bug: 317290555
Change-Id: I944d336e474b9ed61f4554fc9325a8df3a9cf40b