[QNN EP] Fix index-out-of-bounds bug in Slice builder when initializer is shared (#17905)
### Description
There's an index-out-of-bounds bug that is triggered when a Slice
operator shares an initializer with another operator that is processed
first. In this case, QNN EP fails to properly initialize a `raw_starts`
(or `raw_ends`) vector, which is later indexed by a call to
`SliceOp::PrepareForComputeHelper()`.
### Motivation and Context
Fix bug that blocks https://github.com/microsoft/onnxruntime/pull/17764