Rename apiScope.stubsTag

The different roles of stubs (e.g. dependency to java modules,
hiddenapi, dist to sdk snapshot) are now split to different set of stubs
(everything, exportable and eventually runtime), while a single set of
stubs are responsible for all roles for the prebuilt stubs. Thus, the
stubs from source are depending different stubs tag (e.g.
everythingStubsTag, exportableStubsTag), while the prebuilt stubs
utilize stubsTag. Given that this tag is only used for the prebuilt
stubs, it can be renamed to `stubsTag`.

Test: m nothing --no-skip-soong-tests
Bug: 319162970
Change-Id: Iacdf434fab6a403482fd2f1de24e6f260ad80756