Support properly setting and getting LocaleLists

Two setters are added, a hidden one for fast unsafe calls when the
input is known to be OK, and a public one for normal users.

A hidden constructor is also added for adjusting locale lists and
moving a locale to the front.

Finally, an indexOf method is added. It's only used in testing now,
but it's potentially useful for others too.

Bug: 26404035
Change-Id: I4ab3140134325456a8c3df1b126ccc6f8e3f43fd