Support Slice type in SliceData

Add a getter for Slice type for BasePreferenceController.
This facilitates the generation of all possible Slices for Settings
search and discovery for AGSA.

Bug: 62807132
Test: robotests
Change-Id: I2cba1fc72c95769ecc26c76d1f2faa49a285d077

tests/robotests/src/com/android/settings/slices/SliceDataConverterTest.java

@@ -90,5 +90,6 @@ public class SliceDataConverterTest {
         assertThat(fakeSlice.getUri()).isNull();
         assertThat(fakeSlice.getFragmentClassName()).isEqualTo(fakeFragmentClassName);
         assertThat(fakeSlice.getPreferenceController()).isEqualTo(fakeControllerName);
+        assertThat(fakeSlice.getSliceType()).isEqualTo(SliceData.SliceType.SLIDER);
     }
 }