IdlingResource integrations

Compose and Espresso have two different implementations of IdlingResources, we should consider supporting both.

This can be done today by:

1. Creating a class in com.bumptech.glide.load.engine.executor to access GlideExecutor's package protected constructor
2. Passing an instance Espresso's IdlingThreadPoolExecutor class to GlideExecutor
3. Using Glide.init and setting the animation, disk cache and source executors to use the GlideExecutor(s).

The only caveat is that because the GlideExecutor constructor is non-public, we might break this at any point in the future. It would be neater for users if we provided those APIs.