Don't reset channel field when loading the app

Only save data that's changed.

Change-Id: Iaa8265f543a7c1ec418d9d016986e0a253f28df6
Fixes: 79216856
Test: robo tests

tests/robotests/src/com/android/settings/notification/BlockPreferenceControllerTest.java

@@ -250,8 +250,7 @@ public class BlockPreferenceControllerTest {
 
         verify(mBackend, times(2)).updateChannel(any(), anyInt(), any());
 
-        // 2 calls for onSwitchChanged + once when calling updateState originally
-        verify(mBackend, times(3)).setNotificationsEnabledForPackage(
+        verify(mBackend, times(2)).setNotificationsEnabledForPackage(
                 anyString(), anyInt(), anyBoolean());
     }