Use-after free of IMGDNN outputs

[robquill]

ANeuralNetworksExecution_notifyWait calls imgdnnMemoryLock on memory which has already been destroyed in ANeuralNetworksExecution_startCompute.

This is because:

• ANeuralNetworksExecution_setOutput adds to output to execution->imgdnn_memories and execution->host_output_memories
• All memory objects in imgdnn_memories are destroyed at the end of ANeuralNetworksExecution_startCompute
• ANeuralNetworksExecution_notifyWait calls imgdnnMemoryLock on each of the objects in host_output_memories which includes the object previously freed in ANeuralNetworksExecution_startCompute

Fix is to not add outputs to imgdnn_memories and have ANeuralNetworksExecution_notifyWait destroy the output objects.

Fixes OCL: Invalid memory object errors in test_serialize and test_binary.

[robquill]

See #2 for the proposed fix.