[webgpu] use u32 to represent f16 in uniform (#25391)
### Description
For f16 uniform variables, use u32 to bit-wise represent them.
### Motivation and Context
Some devices supports f16 in shader/storage buffer, but not in uniform
buffers. Dawn will set the f16_support to false for them. However, we
don't necessarily have to use f16 in uniform.
This change together with #25349 will enable using f16 models on some
Android devices.
