Fix cache key used by SiteSQLStore.

This makes sure that SiteSQLStore uses a cache key that includes
information about the serialization structure as well as where the
list of sites was loaded from.

This avoids problems with loading "old" serialized versions from the
cache after an upgrade. It also avoids cache conflicts with site lists
loaded from different places.

Change-Id: I7ad5b8ae63dc24598d41b2f150df7c14105d9f62
Merge "Fix cache key used by SiteSQLStore."