add unittests for metrics_reporter_omaha

Test: unittests pass on iot devices
Change-Id: I80f8bf83718ee3ac1b01058076513d1199d027c4
diff --git a/Android.mk b/Android.mk
index 8da8ede..d34898a 100644
--- a/Android.mk
+++ b/Android.mk
@@ -947,6 +947,7 @@
     common_service_unittest.cc \
     fake_system_state.cc \
     image_properties_android_unittest.cc \
+    metrics_reporter_omaha_unittest.cc \
     metrics_utils_unittest.cc \
     omaha_request_action_unittest.cc \
     omaha_request_params_unittest.cc \