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
diff --git a/bpfix/cmd/bpfix.go b/bpfix/cmd/bpfix.go
index f51c6f7..461f41d 100644
--- a/bpfix/cmd/bpfix.go
+++ b/bpfix/cmd/bpfix.go
@@ -75,13 +75,13 @@
 	}
 
 	// compute and apply any requested fixes
-	fixed, err := bpfix.FixTree(file, fixRequest)
+	err = bpfix.FixTree(file, fixRequest)
 	if err != nil {
 		return err
 	}
 
 	// output the results
-	res, err := parser.Print(fixed)
+	res, err := parser.Print(file)
 	if err != nil {
 		return err
 	}