Fix DecimalFormatSymbols.setCurrency.

We cache values from the Currency but can allow those cached values to be
overwritten, so even if a later call to setCurrency is for the same Currency
as last time, we can't assume that's a no-op.

Bug: 18437206
Bug: https://code.google.com/p/android/issues/detail?id=79925
Change-Id: Ic44dbc66a190567438b8b9f866f2a6cdee693e10