[WebNN] Always create a new constant for zero_points (#25286)
MatMulNBits is a decomposed op in WebNN EP. Previously, we share the
WebNN constant of zero_points if they have the same value and data type.
However, this brings a lot of complexity for developers to fuse it back
to MatMulNBits in the underlying WebNN implementation in Chromium.
In this PR, we will always create a new constant for zero_points.