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: android.service.dreams.dreams_v2

Change-Id: Ibfd04cf4e4465fa4193a55b6cf21aca18f0e7e51