Encode command in StreamingCommand

The upstream repo also does similar conversion.
https://github.com/google/python-adb/blob/f4e597fe55900651a8a91fccc1e09061fd96b5e9/adb/adb_protocol.py#L434-L435

This should fix the error in Skia script.
https://console.cloud.google.com/errors/CMiJkO6AgOLHCQ?time=PT6H&project=chrome-swarming

Bug: skia:11875
Change-Id: Ib36b6ac6d430658c8f9120ce96b570beb8f077c3
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/python-adb/+/3112166
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Junji Watanabe <jwata@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>