Deprecate 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 should simplify down to just
one. Mark the POWER_SAVE_MODE_CHANGING broadcast as deprecated in R and
unavailable to apps targeting R. We'll remove it in a future version.

Bug: 79580230
Test: Android builds
Test: atest BatterySaverReceiverTest
Change-Id: I70e20ce53b496a3dc88a1083c230334f53032d81