BatteryStatsImpl: Add parcel methods for DisplayBatteryStats.

DisplayBatteryStats didn't have parcel methods and the estimated
multidisplay screen power was always initialized from 0.
The estimated screen power of the app will become inaccurate
after rebooting.

Bug: 235321975
Test: manual test.
1. Unplug the device.
2. Use an app for a while and keep screen on.
3. Capture the first bug report.
4. Reboot the device.
5. Capture the second bug report.
The estimated screen power of the app shouldn't become lower after rebooting the device.

Author: Shumao Hou <shumao1129@gmail.com>
Change-Id: If74e285102a7350c17c2fc40a205b2031e82f5f4