IVGCVSW-7402 Fix for Native crashes in Arm NNAPI SL

* Instead of extending ArmnnDevice with ArmnnDriver make it a member
* This will allow us to construct the ArmnnDriver and return a
  DEVICE_UNAVAILABLE NN_ERROR instead of segfaulting with the
  armnn::InvalidArgumentException if we cannot create a device

Signed-off-by: Kevin May <kevin.may@arm.com>
Change-Id: Ibbcdbeb33dfd170dd1fb393204620e5b9e6342b8