Gardening M38: Remove classes_dir in gyp files

Upstream removes classes_dir in java.gypi and java_apk.gypi.
Previously, crosswalk relies on it to create xwalk_core_library
and xwalk_runtime_embedded.
For xwalk_core_library, use javac_jar_path instead of classes_dir.
For xwalk_runtime_embedded, the target is not used any more
after xwalk_app_template switch to use xwalk_core_library,
just removed it.
Gardening M38: Append UA commandline at correct time

The nativeAppendSwitch implementation can't be found
until the load and initialization of LibraryLoader is
done, so move the operation to append switch for
device UA string after LibraryLoader.ensureInitialized.
Merge pull request #2346 from wang16/next

Gardening M38 for Android port