Clean up JNI: (...)
* pthread_key_create is used to set a destructor for every thread
created through jni_thread_getenv
* To grab a JNIEnv pointer - go through jni_thread_getenv
* jni_thread_getenv sets pthread_setspecific for the JNIEnv pointer
to bind destructor
* Reuse activity->vm everywhere instead of creating local pointer
copies
* Don't use DetachCurrentThread outside of platform_android's (new)
jni_thread_destruct function - the destructor will do this for us
now