Prevent duplicate dist rules for source and prebuilt C api cov

Adding stub generation support for C prebuilt libraries creates
duplicate dist rules for api coverage files
```
overriding commands for target `<out_dir>/../cc_apis/libc.xml', previously defined at
```

This CL hides the incorrect api coverage file using `IsModulePreferred`.

Bug: 275273834
Test: EMMA_INSTRUMENT=true EMMA_INSTRUMENT_FRAMEWORK=true CLANG_COVERAGE=true NATIVE_COVERAGE_PATHS="*" m droid dist
(no duplicate dist rules)

Change-Id: Id1caade7e80119ed1dfbbef638feb22adf116d63

cc/stub_library.go

@@ -60,7 +60,7 @@ func (s *stubLibraries) GenerateBuildActions(ctx android.SingletonContext) {
 					}
 				}
 			}
-			if m.library != nil {
+			if m.library != nil && android.IsModulePreferred(m) {
 				if p := m.library.getAPIListCoverageXMLPath().String(); p != "" {
 					s.apiListCoverageXmlPaths = append(s.apiListCoverageXmlPaths, p)
 				}