Unlock styles via isPremium default now that Sustainer is discontinued

Per review feedback, the minimal change: default PrefUtils.isPremium to
true (pref_key_premium was never written anywhere, so no device has a
stored value) and make the Style preference row visible in Settings.

Fixes #1771
Fixes #1661

Author: joashrajin

RELEASE-NOTES.txt

@@ -1,5 +1,6 @@
 2.39
 -----
+* Unlocked all styles, including the pure-black AMOLED style, for everyone now that Simplenote Sustainer is discontinued [#1771](https://github.com/Automattic/simplenote-android/issues/1771) [#1661](https://github.com/Automattic/simplenote-android/issues/1661)
 
 2.38
 -----

Simplenote/src/main/java/com/automattic/simplenote/utils/PrefUtils.java

@@ -168,7 +168,7 @@ public static CharSequence versionInfo() {
     }
 
     public static boolean isPremium(Context context) {
-        return getPrefs(context).getBoolean(PREF_PREMIUM, false);
+        return getPrefs(context).getBoolean(PREF_PREMIUM, true);
     }
 
     public static void setIsPremium(Context context, boolean isPremium) {

Simplenote/src/main/res/xml/preferences.xml

@@ -54,7 +54,7 @@
             android:key="pref_key_style"
             android:summary="%s"
             android:title="@string/style"
-            app:isPreferenceVisible="false"
+            app:isPreferenceVisible="true"
             tools:summary="@string/style_classic">
         </Preference>