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
8 files changed