[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
Note that you can also make `GetSignatureFiles` return `const SignatureFileSet` if you want to make it explicit that callers shouldn't modify the returned object.