Fix using aidl files from filegroups

Compute sources including from filegroup and genrule dependencies
before determining if any sources will cause flags to be added.

Test: gen_test.go
Change-Id: I0434b003bbda07a58bb2ce1a0a72997918c8fae2
diff --git a/cc/library.go b/cc/library.go
index 23e4f44..192496a 100644
--- a/cc/library.go
+++ b/cc/library.go
@@ -317,7 +317,7 @@
 	return flags
 }
 
-func (library *libraryDecorator) compilerFlags(ctx ModuleContext, flags Flags) Flags {
+func (library *libraryDecorator) compilerFlags(ctx ModuleContext, flags Flags, deps PathDeps) Flags {
 	exportIncludeDirs := library.flagExporter.exportedIncludes(ctx)
 	if len(exportIncludeDirs) > 0 {
 		f := includeDirsToFlags(exportIncludeDirs)
@@ -325,7 +325,7 @@
 		flags.YasmFlags = append(flags.YasmFlags, f)
 	}
 
-	return library.baseCompiler.compilerFlags(ctx, flags)
+	return library.baseCompiler.compilerFlags(ctx, flags, deps)
 }
 
 func extractExportIncludesFromFlags(flags []string) []string {