Fix override of setCurrencySymbol in DecimalFormatSymbols

The implementation of fromIcuInstance tries to restore currencySymbol
from the ICU instance, but it's not effective because the subsequent
setting of internationalCurrencySymbol overrides the value of
currencySymbol. This patch just changes the order to avoid that.

Also includes a test to verify that overriding the currency symbol in
DecimalFormatSymbols is preserved in a roundtrip through the
DecimalFormat constructor.

Bug: 28732330
Change-Id: I9961459d9c479975e60c305de37daca58effd3cf