convert EmojiPreference and EmojiCompatFont to Kotlin (#1922)

* convert EmojiPreference and EmojiCompatFont to Kotlin

* move preference related to to dedicated preference package

* update proguard-rules.pro

* reformat & add comment

* maintain disposable information in EmojiPreference instead of EmojiCompatFont

app/proguard-rules.pro

@@ -55,6 +55,9 @@
     public static *** v(...);
     public static *** i(...);
 }
+-assumenosideeffects class java.lang.String {
+    public static java.lang.String format(...);
+}
 
 # remove some kotlin overhead
 -assumenosideeffects class kotlin.jvm.internal.Intrinsics {
@@ -62,8 +65,3 @@
     static void checkExpressionValueIsNotNull(java.lang.Object, java.lang.String);
     static void throwUninitializedPropertyAccessException(java.lang.String);
 }
-
-# without this emoji font downloading fails with AbstractMethodError
--keep class * extends android.os.AsyncTask {
-    public *;
-}