[catapult]: Move shell command to its own method.
[catapult]: Move shell command to its own method.

This is in preparation for CL/3652893 for adding a persistent shell,
basically moving the current method for one adb shell command to its own
call. Later an if/else will be added between this _Shell call and a
_persistent_shell call.

Bug: chromium:1021686
Change-Id: I9cb0e14c846ae7b3c7a746fff2cfac6a683224e5
Reviewed-on: https://chromium-review.googlesource.com/c/catapult/+/3803995
Reviewed-by: Ben Pastene <e214258550f76fa23ac7c1d953424f3f468f5c6b@chromium.org>
Commit-Queue: Benjamin Joyce <8df97e251df96fa4a588f28c8657e6dacfbd2aba@chromium.org>
Reviewed-by: Nate Fischer <72afde956856aeb0f00e5e8df797f0068a2ecaf8@chromium.org>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult