Fix kotlin nullable errors in Input tests

Fix kotlin nullable errors that were exposed by setting the retention
of android.annotation.NonNull and android.annotation.Nullable to
class retention.

Bug: 294110802
Test: builds
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:f3413858f14bf7398453603a8df50a8a9169f6a4)
Merged-In: I35530e7ed63a647c39018ef0ee850ceea4dbc0ba

Change-Id: I35530e7ed63a647c39018ef0ee850ceea4dbc0ba

tests/Input/src/com/android/test/input/UnresponsiveGestureMonitorActivity.kt

@@ -45,7 +45,7 @@ class UnresponsiveGestureMonitorActivity : Activity() {
     private lateinit var mInputMonitor: InputMonitor
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
-        val inputManager = getSystemService(InputManager::class.java)
+        val inputManager = checkNotNull(getSystemService(InputManager::class.java))
         mInputMonitor = inputManager.monitorGestureInput(MONITOR_NAME, displayId)
         mInputEventReceiver = UnresponsiveReceiver(
                 mInputMonitor.getInputChannel(), Looper.myLooper())