Change charging string in keyguard

Charging speed won't be meaningful when the battery is in a bad state,
so only provide the general charging info in this case.

Bug: 406894719
Flag: EXEMPT bugfix
Test: local test
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:906118b6e60ba4e152ae8884ec44ec48d3187f0d)
Merged-In: I48f4121bf0cac84cce54cfa32f7ec5b14c0a9a90
Change-Id: I48f4121bf0cac84cce54cfa32f7ec5b14c0a9a90