Implement new design of Bluetooth card

- show only up to 2 connected devices
- show the Bluetooth toggle only when it's off
- remove subtext
- when bluetooth is on but there is no connected device, show the row
  "Pair new device"

Test: robotest
Bug: 142927894
Fixes: 147892635
Change-Id: I1559fc0a4cb0b42bf447d25417ac454c29fe86c3

tests/robotests/src/com/android/settings/testutils/SliceTester.java

@@ -242,6 +242,16 @@ public class SliceTester {
         assertThat(hasText(sliceItems, title, HINT_TITLE)).isTrue();
     }
 
+    /**
+     * Assert no slice item contains title.
+     *
+     * @param sliceItems All slice items of a Slice.
+     * @param title Title for asserting.
+     */
+    public static void assertNoSliceItemContainsTitle(List<SliceItem> sliceItems, String title) {
+        assertThat(hasText(sliceItems, title, HINT_TITLE)).isFalse();
+    }
+
     /**
      * Assert any slice item contains subtitle.
      *