Update time format for the first timestamp on usage chartview.

- If usage data start from the time-change event rather than full-charged event
[Before] https://screenshot.googleplex.com/BokAvKHXmt2Mmwn
[After] https://screenshot.googleplex.com/8thpgVrVt8kqo37

Bug: 336423923
Test: atest SettingsRoboTests:com.android.settings.fuelgauge.batteryusage
Change-Id: I66f8b384938f55852e28bd9f50d1a99c7fc9e41b