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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:74400c273649404b25f950424fdc5a76218a54d5)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:f2d89b00b4fa9ef2cfaedf12b357f2694037302a)
Merged-In: If93d65e6f45271921ef3a013e67ee7ff2bcbed46
Change-Id: If93d65e6f45271921ef3a013e67ee7ff2bcbed46