Use __ANDROID__ instead of __BRILLO__.

__ANDROID__ is defined automatically by the toolchain.

Bug: 23358460
Change-Id: Ib122bfc86a7af6ab6b0b1fcc29dc671e44049524
diff --git a/metricsd/Android.mk b/metricsd/Android.mk
index 0442147..9fd8eda 100644
--- a/metricsd/Android.mk
+++ b/metricsd/Android.mk
@@ -42,7 +42,6 @@
   serialization/serialization_utils.cc
 
 metrics_CFLAGS := -Wall \
-  -D__BRILLO__ \
   -Wno-char-subscripts \
   -Wno-missing-field-initializers \
   -Wno-unused-function \