Add test mapping for CtsUsbSerialTestApp

So that it is only run for the code that might break the test.

Test: TH
Change-Id: I3f749c11260564191bf2220723f040234d40e7fc

services/core/java/com/android/server/os/TEST_MAPPING

@@ -0,0 +1,7 @@
+{
+  "presubmit": [
+    {
+      "name": "CtsUsbTests"
+    }
+  ]
+}