tflite: Use provided tensorflow repo if it exists

This CL also changes the http archive source from .zip to .tar.gz for
consistency.

BUG=b:314854484
TEST=emerge-brya tensorflow

Change-Id: Ifbdc0289393a54ca3a684b7b4ce1e499b823b0b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/tflite/+/5308866
Reviewed-by: Tommy Chiang <ototot@google.com>
Tested-by: Shik Chen <shik@chromium.org>
Commit-Queue: Tommy Chiang <ototot@google.com>