android notif: Parse sender_avatar_url at crunchy shell.

Since at least server v1.6.0, we've never sent a `base_url` --
so the getter `getBaseURL` has never returned anything but `null`.

So, first: hard-wire that to `null`, and constant-fold it through
several layers where a `baseURL` value was passed.

Doing so, we learn in `addHost` that if we'd ever had a
`sender_avatar_url` which didn't start with `http`, we would have
blown up with a NullPointerException on trying to use `baseURL`
to fix it.  This seems to be meant as a way of testing for a
relative URL reference, vs. a real absolute URL (though it's an
awfully buggy one.)

So, pull that check up into the "crunchy shell" of parsing, and
handle it appropriately.  Also replace the check's logic with a
non-buggy version: check that the URL parses as a real, absolute URL.

A question this raises is: how likely was it that we'd get one of
these crash-causing FCM messages (with `sender_avatar_url` not
starting with `http`) in the wild?  In the server Git history, it
looks like server v1.6.0 could send `sender_avatar_url` as a
relative URL if it's a local upload, or if there's no upload and
ENABLE_GRAVATAR is false.  But commit 1.7.0~3388, released Oct
2017, fixed that; since then it's always absolute.

android/app/src/main/java/com/zulipmobile/notifications/FCMPushNotifications.java

@@ -150,7 +150,6 @@ private static Notification.Builder getNotificationBuilder(
         String time = fcmMessage.getTime();
         String stream = fcmMessage.getStream();
         String topic = fcmMessage.getTopic();
-        String baseURL = fcmMessage.getBaseURL();
         int totalMessagesCount = extractTotalMessagesCount(conversations);
 
         if (BuildConfig.DEBUG) {
@@ -173,7 +172,7 @@ private static Notification.Builder getNotificationBuilder(
             }
             if (avatarURL.startsWith("http")) {
                 Bitmap avatar = fetchAvatar(NotificationHelper.sizedURL(context,
-                        avatarURL, 64, baseURL));
+                        avatarURL, 64));
                 if (avatar != null) {
                     builder.setLargeIcon(avatar);
                 }

android/app/src/main/java/com/zulipmobile/notifications/MessageFcmMessage.kt

@@ -1,6 +1,8 @@
 package com.zulipmobile.notifications
 
 import android.os.Bundle
+import java.net.MalformedURLException
+import java.net.URL
 
 
 /**
@@ -43,9 +45,6 @@ internal class MessageFcmMessage private constructor(
     val event: String?
         get() = bundle.getString("event")
 
-    val baseURL: String?
-        get() = bundle.getString("base_url")
-
     protected fun copy(): MessageFcmMessage {
         return fromBundle(bundle)
     }
@@ -64,10 +63,17 @@ internal class MessageFcmMessage private constructor(
                 else -> throw FcmMessageParseException("unexpected recipient_type: $recipientType")
             }
 
+            val avatarURL = bundle.requireString("sender_avatar_url")
+            try {
+                URL(avatarURL)
+            } catch (e: MalformedURLException) {
+                throw FcmMessageParseException("invalid sender_avatar_url: $avatarURL")
+            }
+
             return MessageFcmMessage(
                 email = bundle.requireString("sender_email"),
                 senderFullName = bundle.requireString("sender_full_name"),
-                avatarURL = bundle.requireString("sender_avatar_url"),
+                avatarURL = avatarURL,
 
                 recipientType = recipientType,
                 isGroupMessage = recipientType == "private" && bundle.getString("pm_users") != null,

android/app/src/main/java/com/zulipmobile/notifications/NotificationHelper.java

@@ -45,30 +45,19 @@ static Bitmap fetch(URL url) throws IOException {
         return null;
     }
 
-    static URL sizedURL(Context context, String url, float dpSize, String baseUrl) {
+    static URL sizedURL(Context context, String url, float dpSize) {
         // From http://stackoverflow.com/questions/4605527/
         Resources r = context.getResources();
         float px = TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP,
                 dpSize, r.getDisplayMetrics());
         try {
-            return new URL(addHost(url, baseUrl) + "&s=" + px);
+            return new URL(url + "&s=" + px);
         } catch (MalformedURLException e) {
             Log.e(TAG, "ERROR: " + e.toString());
             return null;
         }
     }
 
-    private static String addHost(String url, String baseURL) {
-        if (!url.startsWith("http")) {
-            if (baseURL.endsWith("/")) {
-                url = baseURL.substring(0, baseURL.length() - 1) + url;
-            } else {
-                url = baseURL + url;
-            }
-        }
-        return url;
-    }
-
 
     private static String extractName(String key) {
         return key.split(":")[0];