Disable proguard enum unboxing optimization

This is broken in proguard v5.1, which is used when building with Gradle

Change-Id: I56670b7e4b4de714f17db43ea492c49d240ff7a4
diff --git a/files/proguard-android-optimize.txt b/files/proguard-android-optimize.txt
index f8f5bcb..3bfe7a5 100644
--- a/files/proguard-android-optimize.txt
+++ b/files/proguard-android-optimize.txt
@@ -10,7 +10,7 @@
 # be complete or up to date. (The "arithmetic" optimization can be
 # used if you are only targeting Android 2.0 or later.)  Make sure you
 # test thoroughly if you go this route.
--optimizations !code/simplification/arithmetic,!code/simplification/cast,!field/*,!class/merging/*
+-optimizations !code/simplification/arithmetic,!code/simplification/cast,!field/*,!class/merging/*,!class/unboxing/enum
 -optimizationpasses 5
 -allowaccessmodification
 -dontpreverify