Adds a FakeAppFocusService to be used by the CarAppFocusManager in tests

This change adds a FakeAppFocusService which can be used by the FakeCar
API when creating the CarAppFocusManager. This allows unit tests for Android
apps to use a (relatively) real CarAppFocusManager.

Test: make -jN RunCarLibTests

Change-Id: Id7aeac302f6631f0578034d3479457d893eb214a
diff --git a/service/src/com/android/car/SystemActivityMonitoringService.java b/service/src/com/android/car/SystemActivityMonitoringService.java
index d192727..b2f2863 100644
--- a/service/src/com/android/car/SystemActivityMonitoringService.java
+++ b/service/src/com/android/car/SystemActivityMonitoringService.java
@@ -268,6 +268,11 @@
             Log.e(CarLog.TAG_AM, "cannot getTasks", e);
             return;
         }
+
+        if (infos == null) {
+            return;
+        }
+
         int focusedStackId = INVALID_STACK_ID;
         try {
             // TODO(b/66955160): Someone on the Auto-team should probably re-work the code in the