[jnigen] Remove FFI patch and use mutex around method/field lookups (https://github.com/dart-lang/jnigen/issues/246)

* Add pubspec.lock from JNI example to git. 
This file should be checked in.

* Remove ffi patch and instead depend on the internals of a pinned package:ffi version (closes https://github.com/dart-lang/jnigen/issues/149)
* Add new tests on GlobalJniEnv
* Rename findJniClass -> findJClass
* Locking around method/field lookups. (closes https://github.com/dart-lang/jnigen/issues/85)
* Rename asJString and asDartString to toJStringPtr and toDartString for uniform naming
* Rename JniClass -> JClass
* Compare bindings normally and fallback to ignoring spaces