If a JNI environment is not already attached to the thread where the
MediaCodec calls are made the current implementation will attach /
detach an environment for each MediaCodec call wasting some CPU time.

ff_jni_get_env replaces ff_jni_{attach,detach} by permanently attaching
an environment (if it is not already the case) to the current thread.
The environment will be automatically detached at the thread destruction
using a pthread_key callback.

Saves around 5% of CPU time (out of 20%) while decoding a stream with
MediaCodec.