Reintroduce support for setting the ICU default locale.

This was presumably inadvertently dropped during a merge.
See go/ag/25653563

Flag: NONE host-only change
Test: m libandroid_runtime, run tests in google3
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:a6a80705bfa49277f0a3e8e6f9b702d77364f8e1)
Merged-In: I8c4765114233f46c9eb1c9fb327f34ca5d131dca
Change-Id: I8c4765114233f46c9eb1c9fb327f34ca5d131dca