update "Last full charge" preference

When there is average time estimation, change it to "Full charge lasts
about" and also update the time.

Also update the BatteryInfo to return average time estimation even
though it is charging.

Bug: 73018395
Test: RunSettingsRoboTests
Change-Id: Ib22dadb5716e3034c60aeeb7768fdbf0cc86be0c