Fix appcenter sdk for public beta

mastodon/proguard-rules.pro

@@ -38,4 +38,10 @@
 -keepclassmembers,allowobfuscation class * {
   @com.google.gson.annotations.SerializedName <fields>;
   @com.squareup.otto.Subscribe <methods>;
-}
+}
+
+-keep class com.microsoft.appcenter.** {
+	*;
+}
+
+-keep class org.joinmastodon.android.AppCenterWrapper { *; }