Java client codegen cleanup (#20691)

* made Configuration.defaultApiClient volatile so that if it's changed the new value is immediately visible to all other threads

* made no-arg api ctor use Configuration.getDefaultApiClient() instead of creating new ApiClient every time

* replaced all use of URLEncoder.encode with ApiClient.urlEncode; just code cleanup; no functional changes

* disabled AbstractJavaCodegenTest.testGeneratedExampleValues as it fails, possibly due to timezone (Locale.ROOT) of local system

* replaced thread-unsafe SimpleDateFormat with DateTimeFormatter and re-enabled unit-test

* removed unused import

* ran the generate-sample scripts and generated a HUGE number of changes!

* some code cleanup and adding booleans to processOpts to remove lots of duplicate FOO.equals(getLibrary()) checks; also using isLibrary() wherever possible

* added missing import of Configuration to api.mustache (and, thus, all native samples)

* added missing import for ApiClient to anyof and oneof model mustaches

* rolled back formatting of cliOptions block (too many changes for a simple PR)

* use toLowerCase with locale

* reverted supportedLibraries reordering and format changes to reduce diff noise

* updated comment

* rolled back Slf4j change due to failing arch-unit test; also rolled back serialization-lib setting code due to unit-test failure (still a mystery)

* moved lib* booleans AFTER super.processOpts() the library can be changed (as shown by unit-test)

Author: ronreynolds