Refactor: Make Hook an abstract class

Reduce visibility of Hook and AsynchronousHook to package private.

Make Hook an abstract class, defining abstract "execute" methods.

Rename the "run" and "submit" methods in SynchronousHook and
AsynchronousHook to "execute" to override that defined in Hook.

Change-Id: I4f69aeb8cce336a5765d784a2a7e793743a33ad7