Corrections in the android part to match changes for desktop.

- Corrected the AndroidLoader, Android does not handle paths with a
trailing /.
- Added missing methods in SurfaceAndroidES20.
Cleaned the Library and resource loaders.

- Finally factorized code used in each resource loader (texture,
shaders, etc.) in trait FileLoader.
- Added a ArmatureLibrary and ArmatureResource to the libraries.
- Added a loader mechanism with a path in Armature.