tflite: Reuse the delegate instance to reduce overhead

BUG=b:342537877
TEST=bazel run --config=host_clang '//tool:op_compat_test' \
       -- --stable_delegate_settings_file=$(realpath xnnpack.json)

Change-Id: Id5cb91c89da32b35a529e18cdf03a364b0415d19
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/tflite/+/5568601
Commit-Queue: Shik Chen <shik@chromium.org>
Tested-by: Shik Chen <shik@chromium.org>
Reviewed-by: Ching-Kang Yen <chingkang@chromium.org>