Correctly test for an object being of the correct type

This `as? T` instruction warns that this is an unchecked cast.
It's unchecked in the literal sense : the compiler actually
doesn't check that the var is of that type. Accordingly, this
will only fail if `it` is null, which never happens.

Concretely what that means is that Java code calling this
method like

  expect(AVAILABLE, network)

...will actually pass for *any* callback matching the network,
because the class is not checked.

Thankfully this code is recent and there doesn't seem to be a
lot of tests that got accepted by this bug when they should
have been rejected.

Test: ConnectivityServiceTest, TH
      new test in this patch that fails before but succeeds after
Change-Id: I2e48dae28ff92045f002cfb2798d383c9c19d5ed

staticlibs/tests/unit/src/com/android/testutils/TestableNetworkCallbackTest.kt

@@ -353,6 +353,13 @@ class TestableNetworkCallbackTest {
         mCallback.expect<Available>(network = hn)
     }
 
+    @Test
+    fun testExpectClass() {
+        val net = Network(1)
+        mCallback.onAvailable(net)
+        assertFails { mCallback.expect(LOST, net) }
+    }
+
     @Test
     fun testPoll() {
         assertNull(mCallback.poll(SHORT_TIMEOUT_MS))

staticlibs/testutils/devicetests/com/android/testutils/TestableNetworkCallback.kt

@@ -236,7 +236,11 @@ open class TestableNetworkCallback private constructor(
         errorMsg: String? = null,
         test: (T) -> Boolean = { true }
     ) = expect<CallbackEntry>(network, timeoutMs, errorMsg) {
-        test(it as? T ?: fail("Expected callback ${type.simpleName}, got $it"))
+        if (type.isInstance(it)) {
+            test(it as T) // Cast can't fail since type.isInstance(it) and type: KClass<T>
+        } else {
+            fail("Expected callback ${type.simpleName}, got $it")
+        }
     } as T
 
     @JvmOverloads