Update Locale to OpenJDK8u60 (part 1/2)

The additions for the new API for RFC 4647 Matching of Language Tags
is left to a potential future part 2/2.

This CL updates all other parts of Locale. Specifically:
* added new hasExtensions() (also used internally by the class),
  stripExtensions() plus corresponding tests
* minor tweaks (formatting, @return sections, default DISPLAY locale
  clarification, ...) to documentation.
* initialize defaultLocale eagerly during <clinit> rather than lazily
* add Android-changed comments to several existing sections that
  were missing them
* introduce field languageTag to cache result of toLanguageTag()
* Note: There are four methods whose implementation on Android is
  based on ICU. Upstream's implementations internally use a helper
  method getDisplayString() that uses LocalServiceProviderPool;
  that method has changed upstream to no longer pass a pool, which
  means that the pool will not be used as a fallback for localizing
  strings. I think the Android implementation doesn't need to change
  but please double check.

Bug: 29935305
Test: cts-tradefed run cts -m CtsLibcoreTestCases -a arm64-v8a

Change-Id: If595cf8b69021f78b76a9a223a0f0d906e74cdc8