Adding "mechanism" and "cell" fields to offline uri. But since
they are not used anywhere, this change sets them to dummy values
so that all the constructors could be updated.
Moved the manifest reconstruction code into a instance class as
the mechanism and cell for the asset is needed in order to create
all the segment uris.
Change-Id: I2b5738805b17c4aa33d39b3166fddf1528aed489
Aaron Vaage