Add lottie_init() and lottie_shutdown() c api.

To support dynamic loading and unloading of rlottie library safely
we need to deallocate the resource cache as well as safely shutdown all the
worker threads.
current patch only stops the Render and Rle task schedulers when lottie_shutdown is called.

Things yet to be implemented during shutdown phase
- Unload image loader if loaded dynamically.
- Check if we can release some cache resources.
- Currently multiple load and unload of rlottie library will not work as we are not starting the
  scheduler again when lottie_init() called multiple time in the same process.