[QNN EP] Fix regression for MatMul with two quantized/dynamic uint16 inputs (#23419)

### Description
- Fixes regression for MatMul with two quantized/dynamic uint16 inputs.
We need to convert input[1] to uint8 to pass QNN validation.
- Separates translation of `ONNX MatMul -> QNN MatMul` and `ONNX MatMul
-> QNN FullyConnected` to separate functions to make the code more
readable.


### Motivation and Context
The following PR updated the handling of MatMul. The logic to handle
MatMul with two non-const uint16 inputs was not ported from
[simple_op_builder.cc](https://github.com/microsoft/onnxruntime/blob/c64fa18834f0651b7d62507a34d802874b099c29/onnxruntime/core/providers/qnn/builder/opbuilder/simple_op_builder.cc#L107)
to the new
[matmul_op_builder.cc](https://github.com/microsoft/onnxruntime/blob/c64fa18834f0651b7d62507a34d802874b099c29/onnxruntime/core/providers/qnn/builder/opbuilder/matmul_op_builder.cc#L57).

#22639

Author: adrianlizarraga