[WebNN] Fix bug in getting the first input name of a WebNN op (#25027)
This util `GetWebNNOpFirstInputName` previously accepted a WebNN op name
as parameter, which will always return "input" because the key of
`op_inputs_map` is ONNX op type.
This PR fixes the bug by:
- Changing the parameter of `GetWebNNOpFirstInputName` to ONNX op
- Changing the `decomposed_op_map` to ONNX-to-ONNX op mapping, in order
to improve the search complexity.