Add the key extraction method in PowerUsageSummary

This cl is to fix the preference flicker issue. The main reason is
that we cannot find cached preference because of the inconsistent
search key. The search key is calculated without considering
uidObj, but the key in preference(aka changed preference) maybe
changed later based on the existence of uidObj. So when we search
the changed preference using the wrong key, we won't get it even
though it exists in the cache.

In my cl, I group two parts(search key calc + considering uidObj)
together in a method to extract the key.

Bug: 33663705
Test: make RunSettingsRoboTests
Change-Id: I2ca0c558ffc0d49ed9576321ecbc02542b568d65