android notif [nfc]: Fold NotificationHelper into NotificationUiManager

There's hardly anything left in this file, after zulip#4842 made it
unnecessary to maintain our own data structure of conversations
because we can use the system's data structures of active
notifications.  Fold what little is left into the main file that
uses it (indeed the only file that uses its one nontrivial item.)

Also add some javadoc while we're at it.

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

@@ -9,6 +9,8 @@ import android.app.NotificationManager
 import android.app.PendingIntent
 import android.content.Context
 import android.content.Intent
+import android.graphics.Bitmap
+import android.graphics.BitmapFactory
 import android.media.AudioAttributes
 import android.net.Uri
 import android.os.Build
@@ -24,6 +26,9 @@ import com.zulipmobile.BuildConfig
 import com.zulipmobile.R
 import com.zulipmobile.ZLog
 import me.leolin.shortcutbadger.ShortcutBadger
+import java.io.IOException
+import java.io.InputStream
+import java.net.URL
 
 // This file maintains the notifications in the UI, using data from FCM messages.
 //
@@ -46,6 +51,12 @@ import me.leolin.shortcutbadger.ShortcutBadger
 // The main entry point is `onReceived`, which is called when we receive
 // an FCM message.
 
+/** An Android logging tag for our notifications code. */
+// (This doesn't particularly belong in this file, but it belongs somewhere
+// in the notifications package.)
+@JvmField
+val TAG = "ZulipNotif"
+
 private val CHANNEL_ID = "default"
 private val NOTIFICATION_ID = 435
 
@@ -210,6 +221,19 @@ private fun extractMessageKey(fcmMessage: MessageFcmMessage): String {
     return messageKey
 }
 
+/** Fetch an image from the given URL.  (We use this for sender avatars.) */
+fun fetchBitmap(url: URL): Bitmap? {
+    return try {
+        val connection = url.openConnection()
+        connection.useCaches = true
+        (connection.content as? InputStream)
+            ?.let { BitmapFactory.decodeStream(it) }
+    } catch (e: IOException) {
+        ZLog.e(TAG, e)
+        null
+    }
+}
+
 /** Handle a MessageFcmMessage, adding or extending notifications in the UI. */
 private fun updateNotification(
     context: Context, fcmMessage: MessageFcmMessage) {

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

@@ -6,7 +6,7 @@
 import com.facebook.react.bridge.*;
 import com.google.firebase.iid.FirebaseInstanceId;
 
-import static com.zulipmobile.notifications.NotificationHelper.TAG;
+import static com.zulipmobile.notifications.NotificationUiManager.TAG;
 
 class NotificationsModule extends ReactContextBaseJavaModule {