Change Locale.getDisplayScript() behavior to default to DISPLAY Locale.

This makes getDisplayScript() more consistent with other methods such
as getDisplayName() and getSymbol() that were already using the default
DISPLAY Locale as opposed to the default Locale.

This updates these methods and their documentation to OpenJDK8u60.

LocaleTest reuses the Locales helper class which was pulled to the
top level from CurrencyTest.Locales and slightly refactored.

Test: libcore.java.util.CurrencyTest
Test: libcore.java.util.LocaleTest

Bug: 29935305

Change-Id: I784269d9695d15873568bad99839d8aadbad4f6c