Modify LOCAL_DROIDDOC_* to export the exportable artifacts

LOCAL_DROIDDOC_STUBS_SRCJAR is used to copy entries to OUT_DOCS in
build/make/core/soong_droiddoc_prebuilt.mk, which defines the copy rules
to generate the docs. When generating the documentation, the exportable
stubs should be set as dependencies, so that disabled flagged apis are
not exposed. Thus, this change modifies the entries passed to
LOCAL_DROIDDOC_STUBS_SRCJAR from the everything stubs to the exportable
stubs.

Likewise, all other LOCAL_DROIDDOC_* entries are copied to dist during
sdk builds. Therefore, this change replaces the everything artifacts to
the exportable artifacts.

This change also config-guards the exportable stubs generation, so that
the exportable stubs are not generated when the droidstubs module does
not generate the stubs.

Test: m docs dist && inspect ninja file to verify that everything stubs are not exported
Bug: 323261972
Change-Id: Ic04fb47ad311aefafc2f1a7b8bb15525050ecd2e