ensure filename to be str

https://console.cloud.google.com/errors/CJHZ5PO1l_zPlwE?service=default&time=PT1H&refresh=auto&project=chromium-swarm-dev
https://console.cloud.google.com/errors/CIWdvZKN5PPObA?time=PT1H&refresh=auto&project=chromium-swarm-dev

I couldn't reproduce "TypeError: must be str, not bytes".
But the error didn't happen with this fix on dev.

Bug: 1198820
Change-Id: I99e49f7713f4771c9bc89e5bf27035cd71a09459
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/python-adb/+/2931748
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Junji Watanabe <jwata@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
2 files changed