Make ResourceKey always use non-null configuration override.

We don't differentiate between null conifguration and
Configuration.EMPTY, but if ResourceKey uses both, it will produce
different hashes and won't be equal. This CL makes ResourceKey always
hold a non null reference to the Configuration object.

We started delivering the configuration overrides in
Ib2c7be0b427f5ce05e7a362bcdd496ddbc9164f0, which changed behavior from
using always null, to using both null and Conifguration.EMPTY for empty
confgiuration. Now we will switch to using only one value, which is
Configuration.EMPTY.

Bug: 22620824
Change-Id: I090fd90ac21a6b3ebc7f2974a91dd7c861af10d7