Update Currency to OpenJDK8u60

- Document existing Android-changed sections that are implemented
  on top of ICU (field icuCurrency).
- Remove documentation that incorrectly stated that Currency data
  can be superseded via a properties file; this is not supported
  on Android.
- In getSymbol() and getDisplayName(), explicitly use
  Locale.getDefault(DISPLAY) rather than rely on ICU Currency's
  no-argument methods doing the same.
- Apply upstream change to turn Map of instances into a
  ConcurrentHashMap.
- Other minor tweaks to minimize diff against upstream.

Bug: 29935305

Test: cts-tradefed run cts -m CtsLibcoreTestCases -a arm64-v8a

Change-Id: I39e0f84c83475f825309b2fecef7b4abad7bd0ff