[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
Merged-In: Ic94da540afa479ed18f1b6fbda4ae3216c37476b
(cherry picked from commit c4f77ac90bf2e48a655ad19b162fe74a23bf3fb0 and
 fix merge conflict in SettingsBackupTest)

tests/net/java/com/android/server/net/NetworkStatsServiceTest.java

@@ -1294,6 +1294,7 @@ public class NetworkStatsServiceTest extends NetworkStatsBaseTest {
         when(mSettings.getPollInterval()).thenReturn(HOUR_IN_MILLIS);
         when(mSettings.getPollDelay()).thenReturn(0L);
         when(mSettings.getSampleEnabled()).thenReturn(true);
+        when(mSettings.getCombineSubtypeEnabled()).thenReturn(false);
 
         final Config config = new Config(bucketDuration, deleteAge, deleteAge);
         when(mSettings.getDevConfig()).thenReturn(config);