Set default value for mobile listPreference

In some cases we don't set value for listPreference, so we need to set
a default one. This is a bug from code migration.

Change-Id: I8072b2a288a6a1254dceabea963f19459b0a9011
Fixes: 119770064
Test: RunSettingsRoboTests

tests/robotests/src/com/android/settings/network/telephony/EnabledNetworkModePreferenceControllerTest.java

@@ -124,6 +124,18 @@ public class EnabledNetworkModePreferenceControllerTest {
         assertThat(mPreference.getSummary()).isEqualTo("3G");
     }
 
+    @Test
+    public void updateState_updateByNetworkMode_useDefaultValue() {
+        Settings.Global.putInt(mContext.getContentResolver(),
+                Settings.Global.PREFERRED_NETWORK_MODE + SUB_ID,
+                TelephonyManager.NETWORK_MODE_LTE_GSM_WCDMA);
+
+        mController.updateState(mPreference);
+
+        assertThat(mPreference.getValue()).isEqualTo(
+                String.valueOf(TelephonyManager.NETWORK_MODE_LTE_GSM_WCDMA));
+    }
+
     @Test
     public void onPreferenceChange_updateSuccess() {
         doReturn(true).when(mTelephonyManager).setPreferredNetworkType(SUB_ID,