Change to public POWER_SAVE_MODE_CHANGED broadcast.

The POWER_SAVE_MODE_CHANGING broadcast is sent at the same time as the
POWER_SAVE_MODE_CHANGED broadcast. Since broadcasts are asynchronous,
there's no reason to have both, so we're simplifying down to just one.

Bug: 79580230
Test: atest BatterySaverReceiverTest
Change-Id: Ib27fc91bc23126f58532e534575d912e3b72ac43