StatsPullAtomCallbackImpl: fixed StrictMode policy violation

- The caller of getBatteryUsageStats must call close() on the resulting BatteryUsageStats object

Flag: com.android.server.power.optimization.add_battery_usage_stats_slice_atom

Bug: 353536815
Test: adb shell cmd stats pull-source 10209; and review logcat for warnings
Change-Id: I038464c66bd1759a29637c0d994be6e09e0f806b