Merge lp://staging/~charlesk/indicator-power/lp-1289465-make-brightness-icons-consistent-with-system-settings into lp://staging/indicator-power/15.04

For the brightness slider, use the same left & right icons as ubunu-system-settings

== Description of the Change

For the brightness slider, use the same left & right icons as ubunu-system-settings.

== Checklist

> Are there any related MPs required for this MP to build/function as expected? Please list.

Yes:
https://code.launchpad.net/~charlesk/ubuntu-themes/lp-1289465-add-display-brightness-icons-to-ubuntu-mobile/+merge/254820

> Is your branch in sync with latest trunk? (e.g. bzr pull lp:trunk -> no changes)

Yes

> Did the code build without warnings?

Yes

> Did the tests run successfully?

Yes

> Did you perform an exploratory manual test run of your code change and any related functionality?

Yes

> If you changed the packaging (debian), did you subscribe the ubuntu-unity team to this MP?

N/A

> Did your component test plan pass? If on a device, what image number?

mako vivid r150

> Please list which manual tests are germane for the reviewer in this MR.

This is a one-off, so putting the test here in the MP:

Steps:
1. Pull down battery indicator's menu
2. Make a note of the icons on the brightness slider
3. Click on "Battery settings..."
4. In the "Battery settings" page, click on "Display brightness"
5. Make a note of the icons on the brightness slider

Expected Results:
The icons in steps 2 and 5 should be the same.

> Did you provide a link to this page https://wiki.ubuntu.com/Process/Merges/Checklists/indicator-power

Yes