Remove POWER_SAVE_MODE_CHANGING broadcast.

The internal 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, especially since it can
potentially lead to inconsistency, so we're simplifying down to just one.

Bug: 79580230
Test: Android builds
Test: atest BatterySaverReceiverTest
Change-Id: I94512aa88f442e827100cc2abf0a02da356641aa