fix CalledFromWrongThreadException in ComposeActivity (#1447)

app/src/main/java/com/keylesspalace/tusky/ComposeActivity.java

@@ -314,6 +314,7 @@ public final class ComposeActivity
                             activeAccount.getFullName()));
 
             mastodonApi.getInstance()
+                    .observeOn(AndroidSchedulers.mainThread())
                     .as(autoDisposable(from(this, Lifecycle.Event.ON_DESTROY)))
                     .subscribe(this::onFetchInstanceSuccess, this::onFetchInstanceFailure);