Fix UiTranslationStateCallback crash due to null locales.

The locales aren't available on a RESUMED event; now they're stored on a
STARTED event. The crash happens because the default Callback impl
converts the Locale to a string to support a deprecated method.

Fix: 186392998
Test: manual - install test apk which triggers the crash. no crash with
 this change.
Change-Id: Iaceca8d29f4c2b0d692c8f6780b0ab963e4e2243