Update mResolvedLocale more aggressively

A client that mutates the locale of the Configuration being updated
can cause failure of the test that mResolvedLocale needs to be
recomputed. This patch stashes the previous locale list privately
so that updates can be detected even in this case.

Bug: 25993843
Change-Id: I1ea17dffd722177fb5b65ee77b9f5ce9a943f15b