[thread] loose the restrictions on ThreadNetworkManager APIs

1. We don't need to assert that ThreadNetworkManager is always null
   when the Thread hardware features are not present. There can be more
   useful Thread features which don't require the hardware
2. The getAllThreadNetworkControllers() API has no documentation says
   the returned list is guaranteed not empty.

Change-Id: Ie77110067ff670053a2125b30b2791b9f54b2582

thread/tests/cts/src/android/net/thread/cts/ThreadNetworkManagerTest.java

@@ -90,21 +90,4 @@ public class ThreadNetworkManagerTest {
 
         assertThat(mManager).isNotNull();
     }
-
-    @Test
-    public void getManager_noThreadFeature_returnsNull() {
-        assumeFalse(mPackageManager.hasSystemFeature("android.hardware.thread_network"));
-
-        assertThat(mManager).isNull();
-    }
-
-    @Test
-    @IgnoreUpTo(Build.VERSION_CODES.TIRAMISU)
-    public void getAllThreadNetworkControllers_managerIsNotNull_returnsNotEmptyList() {
-        assumeNotNull(mManager);
-
-        List<ThreadNetworkController> controllers = mManager.getAllThreadNetworkControllers();
-
-        assertThat(controllers).isNotEmpty();
-    }
 }