Disable proguard for BusinessCard app

Otherwise the com.example.android.businesscard.ContactAccessorSdk*
classes get removed by proguard. This cause a class not found
exception during app launch.

Bug: 62554875
Test: launch the app

Change-Id: Ic3d44b50f51be6514fd6af822717e0864a6126db
diff --git a/samples/BusinessCard/Android.mk b/samples/BusinessCard/Android.mk
index 78eba5f..c2187d6 100644
--- a/samples/BusinessCard/Android.mk
+++ b/samples/BusinessCard/Android.mk
@@ -12,4 +12,6 @@
 
 LOCAL_DEX_PREOPT := false
 
+LOCAL_PROGUARD_ENABLED := disabled
+
 include $(BUILD_PACKAGE)