Settings: Use new PowerManager call to set low power mode.

Bind the master switch to the power manager state instead
of directly to the single setting.

Depends on frameworks/base change:
  Ic030504c9cad9868a7137abbe837b170da37852b

Bug:17460535
Change-Id: I8c68735308f58fdc07d4f64e765e84286918be7a