[js/webgpu] Fix conv2d with activation (#18388)
### Description
Fix #18297
With PR #17766, conv2d activation in mobilenetv2-12 will not be empty.
However, activation is not supported yet in
[biasActivationSnippet](https://github.com/microsoft/onnxruntime/blob/main/js/web/lib/wasm/jsep/webgpu/ops/3rd-party/activation_util.ts#L48C14-L48C36).
This PR makes all places unify to use
[getActivationSnippet](https://github.com/microsoft/onnxruntime/blob/main/js/web/lib/wasm/jsep/webgpu/ops/fuse-utils.ts#L13)
to fix this issue.
