Fix notification background image for RTL devices

The notification background image would not show if the app has
`android:supportsRtl="true"` the `AndroidManifest.xml` AND the device
has a RTL language set. This would result in the notification becoming
a sold color (all known instances have been a white background) and only
 the title and body text showing.

Setting `android:layoutDirection="ltr"` ensures the layout, only used
for images, isn't influenced by the language direction.

This does NOT affect most notifications, only those that use
`android_background_layout` in their notification payload.

This change does NOT affect the rending of the title or body text in any
way.

Author: jkasten2

OneSignalSDK/onesignal/src/main/res/layout/onesignal_bgimage_notif_layout.xml

@@ -7,6 +7,7 @@
     <!--android:textAppearance="@android:style/TextAppearance.StatusBar.EventContent.Title"-->
 
     <RelativeLayout android:id="@+id/os_bgimage_notif_bgimage_align_layout"
+                    android:layoutDirection="ltr"
                     android:layout_width="wrap_content"
                     android:layout_height="64dp"
                     android:paddingLeft="0dp"