Make sure Keep annotation is honored by Proguard.

This fixes a startup crash, caused by the FeatureFactoryImpl class being
renamed by Proguard.

Test: Build and install the app, confirm it no longer crashes.
Bug: 37753652
Change-Id: I881dea033d12ad63e2085a999139c5a4f310bb1a
diff --git a/Android.mk b/Android.mk
index 5159b84..8f65db8 100644
--- a/Android.mk
+++ b/Android.mk
@@ -55,6 +55,8 @@
 
 LOCAL_JACK_COVERAGE_INCLUDE_FILTER := com.android.emergency.*
 
+LOCAL_PROGUARD_FLAG_FILES := proguard.flags
+
 include $(BUILD_PACKAGE)
 
 include $(call all-makefiles-under,$(LOCAL_PATH))