[SB] Don't use conflatedCallbackFlow for BatteryRepository.

conflatedCallbackFlow means that we can drop callback events if too many
come in at one time. Instead, callbackFlow ensures that every callback
event gets processed eventually.

Bug: 433239990
Flag: com.android.systemui.status_bar_battery_no_conflation
Test: atest BatteryRepositoryTest. Verify tests fail with the flag off,
pass with the flag on
Test: smoke test of battery icon

Change-Id: I849221569b46124395f7e060e0d5244bd3f02b08