Add descriptions to reader prev/next buttons for a11y

app/src/main/res/layout/reader_activity.xml

@@ -65,6 +65,7 @@
                 android:layout_width="wrap_content"
                 android:layout_height="wrap_content"
                 android:background="?selectableItemBackgroundBorderless"
+                android:contentDescription="@string/action_previous_chapter"
                 android:padding="@dimen/material_layout_keylines_screen_edge_margin"
                 app:srcCompat="@drawable/ic_skip_previous_white_24dp" />
 
@@ -95,6 +96,7 @@
                 android:layout_width="wrap_content"
                 android:layout_height="wrap_content"
                 android:background="?selectableItemBackgroundBorderless"
+                android:contentDescription="@string/action_next_chapter"
                 android:padding="@dimen/material_layout_keylines_screen_edge_margin"
                 app:srcCompat="@drawable/ic_skip_next_white_24dp" />