Enable droidstubs to export "exportable" artifacts

This change adds support for the droidstubs module to export the
"exportable" artifacts via OutputFiles(tag string) method, while
supporting the current behavior of exporting "everything" artifacts via
a tag. With the support, a rdep module can depend on the "exportable"
(and "runtime" in the long run) artifacts by prepending the stubs type
in the tag. For instance, given that the currently supported tag
{.annotations.zip} exports the everything annotations.zip file,
{.exportable.annotations.zip} tag will export the exportable
annotations.zip file. For an unsupported combination (e.g. all runtime
stubs related artifacts as of now), an error will be thrown.

Test: m nothing --no-skip-soong-tests
Bug: 315490657
Change-Id: Idcefd9cdc02d323306fb8d7be2a2b34f67501f56