Don't pretend *parser.List is immutable

The functions in bpfix that take a *parser.List and return a
modified *parser.List are always returning the same pointer
and mutating the target of the pointer.  Remove the extra
unnecessary return value.

Also extract the getLiteralListProperty function.

Test: androidmk_test.go
Change-Id: I08d8aff955c72b7916741cda8083974a49af4d6f

androidmk/cmd/androidmk/androidmk.go

@@ -239,12 +239,12 @@ func convertFile(filename string, buffer *bytes.Buffer) (string, []error) {
 	}
 
 	// check for common supported but undesirable structures and clean them up
-	fixed, err := bpfix.FixTree(tree, bpfix.NewFixRequest().AddAll())
+	err := bpfix.FixTree(tree, bpfix.NewFixRequest().AddAll())
 	if err != nil {
 		return "", []error{err}
 	}
 
-	out, err := bpparser.Print(fixed)
+	out, err := bpparser.Print(tree)
 	if err != nil {
 		return "", []error{err}
 	}