Create a dummy MetricsReporterStub class

This handles the update_engine_sideload target, and thus avoiding
the insertion of "#ifdef _UE_SIDELOAD" everywhere when we report the
metrics for android.

Bug: 30989466
Test: build pass on marlin
Change-Id: Ib00ab8476218df15768bdbae1a3510f8584aa7c7
diff --git a/Android.mk b/Android.mk
index cc06643..f02794c 100644
--- a/Android.mk
+++ b/Android.mk
@@ -466,7 +466,7 @@
 LOCAL_SRC_FILES := \
     boot_control_recovery.cc \
     hardware_android.cc \
-    metrics_reporter_android.cc \
+    metrics_reporter_stub.cc \
     metrics_utils.cc \
     network_selector_stub.cc \
     proxy_resolver.cc \