Remove usages of namespace in generate_jni in BUILD.gn files

This CL removes usages of namespace in generate_jni, and use
@JNINamespace("foo") instead.

Bug: 396707350
Change-Id: Ia6d76760a22cf00cb42037ec4761a35b4c2fec36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7363020
Owners-Override: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Martin Kong <martinkong@google.com>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1567791}
NOKEYCHECK=True
GitOrigin-RevId: 45a5f753d943301d61a73c3f69d9e16f206f2f3e