SettingsProvider: dup-suppress from cache.

On insert(), check to see if the value is redundant by checking if
it's the same value already in our cache (but without faulting it in
to check).  If so, avoid hitting sqlite or spamming all the
notification listeners with such uselessness.

This reportedly is happening a fair bit.

Change-Id: If58feb3ff1d00027dd927e0900087388cbcd72ae