Add `dumpsys batterystats --sync` command for use in tests

This command can be used in tests to ensure that all pending
batterystats operations, including usage stats aggregation,
are completed.

Bug: 409640329
Test: adb shell dumpsys batterystats --sync
Flag: EXEMPT bug fix
Change-Id: Ie3e5af98b8892925979bf0eb31ff1027c0722e5a