IVGCVSW-1998 : replace Compute enum in LayerSupport free functions

!android-nn-driver:153490

Change-Id: I1c2a5f942e3a1c3626e093c90545ca27c64ba5e8
diff --git a/src/backends/reference/RefBackend.cpp b/src/backends/reference/RefBackend.cpp
index ef52a5e..1f08d82 100644
--- a/src/backends/reference/RefBackend.cpp
+++ b/src/backends/reference/RefBackend.cpp
@@ -15,10 +15,9 @@
 
 namespace
 {
-const BackendId s_Id{"CpuRef"};
 
 static BackendRegistry::Helper s_RegisterHelper{
-    s_Id,
+    RefBackend::GetIdStatic(),
     []()
     {
         return IBackendUniquePtr(new RefBackend, &RefBackend::Destroy);
@@ -27,8 +26,9 @@
 
 }
 
-const BackendId& RefBackend::GetId() const
+const BackendId& RefBackend::GetIdStatic()
 {
+    static const BackendId s_Id{"CpuRef"};
     return s_Id;
 }