[autotest] Implement several adb_host functions.

Add in:
- send_file()
- get_file()
- get_release_version()

BUG=chromium:532226
TEST=Extend dummy pass server test on moblab to {get,send}file and
     get the version successfully from/to a beaglebone/dragonboard.

Change-Id: I1342ce39042517c03b9de1a39f7b810db01dcbbc
Reviewed-on: https://chromium-review.googlesource.com/301143
Commit-Ready: Kevin Cheng <kevcheng@chromium.org>
Tested-by: Kevin Cheng <kevcheng@chromium.org>
Reviewed-by: Simran Basi <sbasi@chromium.org>
1 file changed