Add internal power mode changed broadcast

This broadcast allows settings to be (rarely) be started by a power
change event.

Settings will have a conditional item informing the user of battery
saving mode, when they dismiss it, the item will not show up again
until after power saving mode has been exited and re-entered.
Settings will catch the exit of power saving mode by enabling a
receiver for this broadcast only when in this state.

Change-Id: I62385413e4b8b004d4e1e0e5ba250730848ba013