[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
The caller is not the same namespace with this function. Do you mean I add const with this function and    in the caller part use const to declare. So the stutcure is as below:
const SignatureFileSet GetSignatureFiles(const base::FilePath& widget_path) {}
 SignatureValidator::Status SignatureValidator::Check() {
...
const SignatureFileSet signature_set = GetSignatureFiles(widget_path). 
...
}
Right?