Fix wrong index being used for constant tensor

The delegate still uses a predefined index to get the tensor with the
constant data which isn't guaranteed to be correct because TFLite will
reorder the inputs. It has now been changed to use the index retrieved
during the prepare stage to ensure the correct tensor is used.

Change-Id: Ia3a930a0a61a7e9094ba3e671a2c297d423c2f7c
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>