Update charging string to match keyguard.

Use the keyguard algorithm for computing the charging string in the
low-light clock dream, so that the same string appears in both keyguard
and the low-light clock.

Fixes: 424297980
Test: atest ChargingStatusProviderTest, KeyguardIndicationControllerTest
Flag: com.android.systemui.lowlight_clock_uses_keyguard_charging_status
Change-Id: If93d65e6f45271921ef3a013e67ee7ff2bcbed46