Add scrim behind Reader fullscreen photo viewer toolbar (#23126)

The back arrow was invisible over white/light images because the toolbar
is transparent and its icon is always white. Add a dark gradient scrim
behind the toolbar so the arrow stays legible regardless of image color.

Author: nbradbury

WordPress/src/main/res/drawable/reader_photo_viewer_toolbar_scrim.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape xmlns:android="http://schemas.android.com/apk/res/android">
+    <gradient
+        android:angle="90"
+        android:startColor="@android:color/transparent"
+        android:endColor="@color/black_translucent_50"
+        android:type="linear" />
+</shape>

WordPress/src/main/res/layout/reader_activity_photo_viewer.xml

@@ -32,6 +32,7 @@
         android:id="@+id/toolbar"
         android:layout_width="match_parent"
         android:layout_height="?attr/actionBarSize"
+        android:background="@drawable/reader_photo_viewer_toolbar_scrim"
         android:elevation="@dimen/appbar_elevation"
         app:contentInsetEnd="@dimen/toolbar_content_offset_end"
         app:contentInsetLeft="@dimen/toolbar_content_offset"