Load post privacy preference

This queries the user's post visibility preference when opening the composer, and sets it on the composer.

In the case of composing a reply, the user's preference is only respected if it is "more private" than the privacy of the post being replied to, as this appears to be the behaviour in the web interface (and is what I'd expect)

mastodon/src/main/java/org/joinmastodon/android/api/requests/accounts/GetPreferences.java

@@ -0,0 +1,10 @@
+package org.joinmastodon.android.api.requests.accounts;
+
+import org.joinmastodon.android.api.MastodonAPIRequest;
+import org.joinmastodon.android.model.Preferences;
+
+public class GetPreferences extends MastodonAPIRequest<Preferences> {
+    public GetPreferences(){
+        super(HttpMethod.GET, "/preferences", Preferences.class);
+    }
+}