Remove isValid() from public API.

* Remove isValid() from VendorManifest; rather,
  add(ManifestHal &&) will do the check.
* Make ManifestHal::isValid() private; its XML Converter
  and string parser will do the check.

Test: libvintf_test
Bug: 34620633
Change-Id: I3c8164e5856ac06eb6d79bb34e092ec2dec4e68e
diff --git a/parse_string.cpp b/parse_string.cpp
index b0c8657..f8f4b12 100644
--- a/parse_string.cpp
+++ b/parse_string.cpp
@@ -174,7 +174,7 @@
     if (!parse(v[5], &hal->versions)) {
         return false;
     }
-    return true;
+    return hal->isValid();
 }
 
 std::ostream &operator<<(std::ostream &os, const ManifestHal &hal) {