tflite: log: Fix vlog level parsing

The pointers should point to the stored strings, instead of the
temporary copied values.

BUG=b:305999697,b:340207771
TEST=bazel run --config=host_clang '//common:log_test'

Change-Id: Ied8b3080af535328eb7b3250fe998344abcc8469
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/tflite/+/5535489
Tested-by: Shik Chen <shik@chromium.org>
Commit-Queue: Shik Chen <shik@chromium.org>
Reviewed-by: Yi Chou <yich@google.com>