remove stub dependency on source_library

Since the source_library attribute of _cc_stub_library_shared was a
label attribute, the source library was added as a dependency of the
stub which doesn't always make sense. E.g. it caused validation actions
for the source library to run even when we weren't building the source
library.

This change converts the label attriubte to a string attribute so that
we don't add the dependency.

Bug: 263390551
Change-Id: I19c62d1e14847dff68ab37156452addaa119eaf9