[jnigen] `JniX` to `JX` rename (https://github.com/dart-lang/jnigen/issues/126) Closed https://github.com/dart-lang/jnigen/issues/124. Closed https://github.com/dart-lang/jnigen/issues/125. * Added `JPrimitive` and its subclasses instead of using the typedefs from `ffi` directly. * Renamed `JniObject`, `JniString`, and ... to the shorter `JObject`, JString`, and ... * Made tests more robust by deleting the objects in an arena.