Fix last seen divider crash on Android <5.0

Apparently, it's not possible to reference an attribute in an xml
drawable on API <21, so we have to use separate light and dark theme
drawables instead.

Closes #6285

res/drawable/last_seen_divider_text_background_dark.xml

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <shape xmlns:android="http://schemas.android.com/apk/res/android" android:shape="rectangle" >
-    <solid android:color="?conversation_item_last_seen_text_background"/>
+    <solid android:color="#ff333333"/>
 
     <corners android:radius="65dp"/>
     <padding android:bottom="10dp" android:left="15dp" android:right="15dp" android:top="10dp"/>

res/drawable/last_seen_divider_text_background_light.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape xmlns:android="http://schemas.android.com/apk/res/android" android:shape="rectangle" >
+    <solid android:color="@color/white"/>
+
+    <corners android:radius="65dp"/>
+    <padding android:bottom="10dp" android:left="15dp" android:right="15dp" android:top="10dp"/>
+</shape>

res/layout/conversation_item_last_seen.xml

@@ -15,6 +15,6 @@
                   android:textSize="12sp"
                   android:textAllCaps="true"
                   android:textStyle="bold"
-                  android:background="@drawable/last_seen_divider_text_background"
+                  android:background="?conversation_item_last_seen_text_background"
                   tools:text="3 unread messages" />
 </FrameLayout>

res/values/attrs.xml

@@ -73,7 +73,7 @@
     <attr name="conversation_item_sent_indicator_text_background" format="reference" />
     <attr name="conversation_item_header_background" format="reference"/>
     <attr name="conversation_item_last_seen_background" format="reference|color"/>
-    <attr name="conversation_item_last_seen_text_background" format="color"/>
+    <attr name="conversation_item_last_seen_text_background" format="reference"/>
 
     <attr name="dialog_info_icon" format="reference" />
     <attr name="dialog_alert_icon" format="reference" />

res/values/themes.xml

@@ -158,7 +158,7 @@
         <item name="conversation_item_received_text_secondary_color">#BFffffff</item>
         <item name="conversation_item_header_background">@drawable/conversation_item_header_background</item>
         <item name="conversation_item_last_seen_background">@drawable/last_seen_background</item>
-        <item name="conversation_item_last_seen_text_background">@color/white</item>
+        <item name="conversation_item_last_seen_text_background">@drawable/last_seen_divider_text_background_light</item>
 
         <item name="quick_camera_icon">@drawable/quick_camera_light</item>
         <item name="quick_mic_icon">@drawable/ic_mic_grey600_24dp</item>
@@ -247,7 +247,7 @@
         <item name="conversation_item_sent_indicator_text_background">@drawable/conversation_item_sent_indicator_text_shape_dark</item>
         <item name="conversation_item_header_background">@drawable/conversation_item_header_background_dark</item>
         <item name="conversation_item_last_seen_background">#66333333</item>
-        <item name="conversation_item_last_seen_text_background">#ff333333</item>
+        <item name="conversation_item_last_seen_text_background">@drawable/last_seen_divider_text_background_dark</item>
 
         <item name="verification_background">#ff333333</item>