Improve battery time calculations

[kylecorry31]

• [ ] Store and update average battery percent increase/decrease per hour
• [ ] Store and update average battery capacity (mAh) increase/decrease per hour
• [ ] Store and update maximum battery capacity (mAh)
• [ ] Show battery times for average usage
• [ ] Show battery times for current usage
• [ ] Add PowerSubsystem.getChargeTime(percent, current?)
• [ ] Add PowerSubsystem.getDischargeTime(percent, current?)
• [ ] Add PowerSubsystem.getTimeUntilDead(current?)
• [ ] Add PowerSubsystem.getTimeUntilFull(current?)
• [ ] Add PowerSubsystem.getBatteryPercentage()

Either the PowerSubsystem will update at a fixed interval (battery change events are too frequent) or the methods which need battery percentage will be suspend functions.

getChargeTime

• percent: The percentage points to charge
• current: If supplied, use the current to estimate the charge time

getDischargeTime

• percent: The percentage points to discharge
• current: If supplied, use the current to estimate the discharge time

getTimeUntilFull (suspend, or cached)

• current: If supplied, use the current to estimate the charge time

getTimeUntilEmpty (suspend, or cached)

• current: If supplied, use the current to estimate the discharge time

getBatteryPercentage (suspend, or cached)