Add possibility to use custom native library loader.

[Tapchicoma]

This changes allow to implement custom strategy for loading JNI libraries and do some fallback in case of error.

For example users can use ReLinker.

[developernotes]

Hi @Tapchicoma

This is an interesting idea, do you have thoughts on an alternative LibraryLoader implementation? All current work is occurring in the android-n-preview branch, not the master at the moment. We ask that you submit a contributor agreement in order for us to accept a request. More information about this agreement can be found here.

[Tapchicoma]

Hi @developernotes.

I signed agreement.

The main issue for me is to use this ReLinker library so my app will be more failsafe on app updates. You can read their blog post about it here

[developernotes]

Hi @Tapchicoma

I apologize for the delay, we've just merged this into master. Thanks!