Improve stability of test BatteryUsageStatsTests#testBatteryUsageStatsPerUid

- add `dumpsys batterystats --sync` to ensure battery usage stats
  accumulation is completed before the atom is pulled

Flag: EXEMPT TEST_ONLY
Bug: 409640329
Test: atest android.cts.statsdatom.batterystats.BatteryUsageStatsTests#testBatteryUsageStatsPerUid
Change-Id: I99d2bceaab8ab3d11634f5313e4a6f50f8ae2e58