Fix kotlin nullable errors in TrustTests

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
Change-Id: I31c4f4d256ff8dd2b6ed1f6ed74844ccaf7a4814

tests/TrustTests/src/android/trust/test/lib/LockStateTrackingRule.kt

@@ -30,7 +30,7 @@ import org.junit.runners.model.Statement
  */
 class LockStateTrackingRule : TestRule {
     private val context: Context = getApplicationContext()
-    private val windowManager = WindowManagerGlobal.getWindowManagerService()
+    private val windowManager = checkNotNull(WindowManagerGlobal.getWindowManagerService())
 
     @Volatile lateinit var lockState: LockState
         private set

tests/TrustTests/src/android/trust/test/lib/ScreenLockRule.kt

@@ -36,7 +36,7 @@ import org.junit.runners.model.Statement
 class ScreenLockRule : TestRule {
     private val context: Context = getApplicationContext()
     private val uiDevice = UiDevice.getInstance(getInstrumentation())
-    private val windowManager = WindowManagerGlobal.getWindowManagerService()
+    private val windowManager = checkNotNull(WindowManagerGlobal.getWindowManagerService())
     private val lockPatternUtils = LockPatternUtils(context)
     private var instantLockSavedValue = false