Handle cc_defaults' `shared` for cc_library_shared

Akin to testing for cc_defaults' `static`, we introduce
a `shared` sibling to
TestStaticLibrary_SystemSharedLibsStaticEmpty

Bug: 195451854
Test: TestCcLibrarySharedSystemSharedLibsSharedEmpty
Change-Id: I63cb0641d216e8c11586ecf415fa2441e31dd3e5