Add proguard flags from "android-proguard.txt"

This resolves an issue where proguard would remove the #onButtonClick
method from the Calculator activity.

Change-Id: I82ed2154c422b43ca147316571acf845c05b3d61
diff --git a/proguard.flags b/proguard.flags
index dfd6702..1185cca 100644
--- a/proguard.flags
+++ b/proguard.flags
@@ -1,4 +1,56 @@
+# This is a configuration file for ProGuard based on "proguard-android.txt".
+# http://proguard.sourceforge.net/index.html#manual/usage.html
+
+-dontusemixedcaseclassnames
+-dontskipnonpubliclibraryclasses
+-verbose
+
+# Optimization is turned off by default. Dex does not like code run
+# through the ProGuard optimize and preverify steps (and performs some
+# of these optimizations on its own).
+-dontoptimize
+-dontpreverify
+
+-keepattributes *Annotation*
+-keep public class com.google.vending.licensing.ILicensingService
+-keep public class com.android.vending.licensing.ILicensingService
+
+# For native methods, see http://proguard.sourceforge.net/manual/examples.html#native
+-keepclasseswithmembernames class * {
+    native <methods>;
+}
+
+# Keep setters in Views so that animations can still work.
+# see http://proguard.sourceforge.net/manual/examples.html#beans
+-keepclassmembers public class * extends android.view.View {
+   void set*(***);
+   *** get*();
+}
+
+# Keep methods in Activity that could be used in the XML attribute onClick.
+-keepclassmembers class * extends android.app.Activity {
+   public void *(android.view.View);
+}
+
+# For enumeration classes, see http://proguard.sourceforge.net/manual/examples.html#enumerations
+-keepclassmembers enum * {
+    public static **[] values();
+    public static ** valueOf(java.lang.String);
+}
+
+-keep class * implements android.os.Parcelable {
+  public static final android.os.Parcelable$Creator *;
+}
+
+-keepclassmembers class **.R$* {
+    public static <fields>;
+}
+
+# The support library contains references to newer platform versions.
+# Don't warn about those in case this app is linking against an older
+# platform version.  We know about them, and they are safe.
+-dontwarn android.support.**
+
 # Some small BoundedRational methods like equals() are not used by the
 # calculator, but crucial for testing.
-
 -keepclassmembers class com.android.calculator2.BoundedRational { *; }