SettingConfirmationHelper: Create internal UiContext to load themed resources

As for Theme Engine introduction, we have to handle context changes. In this case,
a wrong context is passed from VolumePanel to SettingConfirmationHeHlper to be able
to write to system. We fixed it by sending a system context separately from VolumePanel
to let it write to system. Although, right now Safe Headset dialog isn't themed because it
doesn't receive the theme context.

Fix that by creating a new UiContext inside SettingConfirmationHelper, that we will use
just to load themed resources.

Continue this https://gerrit.paranoidandroid.co/#/c/4452/

Signed-off-by: Carlo Savignano <stevewatersy@gmail.com>