Return null when setting can't be changed.

If a Slice cannot be changed, return null instead of
an error slice.

Bug: 80155832
Test: robotestst
Change-Id: Ib94136c449c6d9c1911f89833bba62fd2263daa4

tests/robotests/src/com/android/settings/wifi/calling/WifiCallingSliceHelperTest.java

@@ -114,7 +114,7 @@ public class WifiCallingSliceHelperTest {
         final Slice slice = mWfcSliceHelper.createWifiCallingSlice(mWfcURI);
 
         testWifiCallingSettingsUnavailableSlice(slice, null,
-                SliceBuilderUtils.getSettingsIntent(mContext));
+                WifiCallingSliceHelper.getSettingsIntent(mContext));
     }
 
     @Test
@@ -125,7 +125,7 @@ public class WifiCallingSliceHelperTest {
 
         assertThat(mWfcSliceHelper.getDefaultVoiceSubId()).isEqualTo(1);
         testWifiCallingSettingsUnavailableSlice(slice, null,
-                SliceBuilderUtils.getSettingsIntent(mContext));
+                WifiCallingSliceHelper.getSettingsIntent(mContext));
     }
 
     @Test