[SM07] Make combine subtype configurable from Settings

Note that enabling/disabling would not take effect until device
reboot. This will be addressed in follow-up patch.

Test: 1. atest NetworkStatsServieTest SettingsBackupTest
      2. adb shell settings put global netstats_combine_subtype_enabled 1|0
Bug: 146415925

Change-Id: Ic94da540afa479ed18f1b6fbda4ae3216c37476b