IVGCVSW-2454 Refactor ArmNN to support pluggable backends from a separate

IVGCVSW-2454 Refactor ArmNN to support pluggable backends from a separate
code base

 * Made the virtual functions in ILayerSupport.hpp pure
 * Created a LayerSupportBase class with the default implementation of
   the interface
 * Made the backend layer support classes inherit from the base
   class, instead of directly from the interface
 * Refactored the profiler and the profiling event classes to use
   the BackendId instead of the Compute
 * Implemented a proper MemCopy support method
 * Changed Compute to BackendId in the profiling API and objects
 * Removed static references to pluggable backends

!android-nn-driver:492

Change-Id: Id6332b5f48c980819e0a09adc818d1effd057296