Make single shell call in _SetScalingGovernorInternal

Make single shell call in _SetScalingGovernorInternal

Changed implementation to make a single shell call, rather than
pushing files and executing several shell calls.

When running telemetry unittests on android, reduces total execution
time about 12%.

BUG=379378
Committed: https://crrev.com/4bda9f96e6edbc4c208abde787c842f828f2f798
Cr-Commit-Position: refs/heads/master@{#292405}

[perezju, 2014-08-28 15:51:44]

perezju@chromium.org changed reviewers:
+ skyostil@chromium.org

[Sami, 2014-08-28 15:57:44]

Nice, the change itself lgtm, but one nit about the changelog message: the first
line should be a title (up to 50 chars), followed by a blank line, followed by
the change description.

[perezju, 2014-08-28 16:08:24]

done

[perezju, 2014-08-28 16:08:49]

The CQ bit was checked by perezju@chromium.org

[commit-bot: I haz the power, 2014-08-28 16:09:58]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/perezju@chromium.org/514123005/1

[commit-bot: I haz the power, 2014-08-28 17:07:36]

Committed patchset #1 (id:1) as 1b65bc5728b25e4017d66f88a1987a9f1000971d

[commit-bot: I haz the power, 2014-09-10 03:00:18]

Patchset 1 (id:??) landed as
https://crrev.com/4bda9f96e6edbc4c208abde787c842f828f2f798
Cr-Commit-Position: refs/heads/master@{#292405}