According to

https://stackoverflow.com/questions/46233200/stop-buffering-of-adb-shell-output

the adb exec-out commands avoids the buffering inherent to adb shell.

Let's see if using exec-out will fix the android builder flakyness where
exitcodes or output were sometimes missing.

Updates #30512 (perhaps fixes it).

Change-Id: Ib953ef0262b20730e0d4c332058d29c5066bfeb2
Reviewed-on: https://go-review.googlesource.com/c/164661
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>