[Tizen] Fix bug on intalling widget failed

It is caused by temp value in local function used by outer function.
And for contructor of WGTPACKGE should set is_valid_ to false when
error happens.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2403
SignatureFileSet GetSignatureFiles(const base::FilePath& widget_path) {}

const SignatureFileSet& signature_set = GetSignatureFiles(widget_path).