Add shared strings to settingsLib

Now that hybrid notifications exist in SysUI these strings should
be moved into settingsLib so we don't duplicate them. Additionally
several utility methods were moved so that they could be shared to
help keep behavior and strings consistent between different system
components that show battery time remaining.

Test: robotests
Bug: 65656091
Bug: 66909350
Bug: 67469159
Change-Id: Ifdaf02cf39354ed0088560490eabf8b101788a83