COMPMID-1478: Stop relying on static default OpenCL objects in cl2.hpp

This causes problems when ACL is used as a shared library on Android.
Fixes some problems related to creation / destruction order between the Graph's CL backend and core / runtime

Change-Id: I716d63fd42f4586df1ffbb6fa97e4db06d3a781b
Reviewed-on: https://eu-gerrit-1.euhpc.arm.com/143228
Tested-by: Jenkins <bsgcomp@arm.com>
Reviewed-by: Michele DiGiorgio <michele.digiorgio@arm.com>
Reviewed-by: Gian Marco Iodice <gianmarco.iodice@arm.com>
diff --git a/src/graph/backends/NEON/NEDeviceBackend.cpp b/src/graph/backends/NEON/NEDeviceBackend.cpp
index 7c2db40..5fc44d0 100644
--- a/src/graph/backends/NEON/NEDeviceBackend.cpp
+++ b/src/graph/backends/NEON/NEDeviceBackend.cpp
@@ -61,6 +61,13 @@
 
 void NEDeviceBackend::initialize_backend()
 {
+    //Nothing to do
+}
+
+void NEDeviceBackend::release_backend_context(GraphContext &ctx)
+{
+    //Nothing to do
+    ARM_COMPUTE_UNUSED(ctx);
 }
 
 void NEDeviceBackend::setup_backend_context(GraphContext &ctx)
@@ -155,4 +162,4 @@
 }
 } // namespace backends
 } // namespace graph
-} // namespace arm_compute
\ No newline at end of file
+} // namespace arm_compute