Add isSliceable API to BasePrefController

Only support explicitly approved Settings Slices,
dictated by controllers which return true for the new
method isSliceable.

Updating the supported settings to a whitelist means that
the method to return all available slices must be updated,
and checking slicability when we index slices.

Test: robotests
Change-Id: I85848c2cdf3e151fa94b33dd1dc5c0374ef94b5b
Merged-In: Ib2b9690cdd0036b5cc4a1cb846c52bce7c824ab9
Fixes: 79779103

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

@@ -97,4 +97,11 @@ public class RingVolumePreferenceControllerTest {
     public void getAudioStream_shouldReturnRing() {
         assertThat(mController.getAudioStream()).isEqualTo(AudioManager.STREAM_RING);
     }
+
+    @Test
+    public void isSliceableCorrectKey_returnsTrue() {
+        final RingVolumePreferenceController controller =
+                new RingVolumePreferenceController(mContext);
+        assertThat(controller.isSliceable()).isTrue();
+    }
 }