[Tizen] Add installer update signal support

To support "update" signal, a new "--update" argument is added to
xwalk-pkg-helper. The PkgInstaller is also refactored, original
install/uninstall methods are made internal, the public
install/uninstall/update methods will handle these signals sending.

BTW, as the package helper is only used by Tizen, remove Linux build for
it.

BUG=XWALK-1600