Additional cleanup prior to adding the runtime stubs

This change performs additional cleanups in the build rules of the
droidstubs module, specifically optionalizing annotationFlags(...) and
apiLevelsAnnotationsFlags(...). This is required as "runtime" stubs
would not generate other artifacts than the stubs, and thus
`stubsArtifacts` would not be generated for "runtime".

Test: m nothing
Bug: 319162970
Change-Id: Ia009df3d01f49841d5041fb2dfdfa7ef8671bf18