Add a confirmation dialog to empty trash/spam.

Fixes b/8753294. Creates a new EmptyFolderDialogFragment
that displays the dialog, handles orientation changes, and
calls back to AbstractActivityController when delete is selected
to perform the mass deletion. Additionally, we now only show
the empty overflow options if there are conversations in the
folder.

Change-Id: Ibc9ff0b9511a4cdc4c390cd7273706abedb8d9a2
diff --git a/res/values/strings.xml b/res/values/strings.xml
index 87a10af..7b6f05e 100644
--- a/res/values/strings.xml
+++ b/res/values/strings.xml
@@ -881,10 +881,22 @@
     <string name="checking_for_mail">Checking for mail</string>
 
     <!-- Conversation list screen overflow menu in trash folder [CHAR LIMIT=50]  -->
-    <string name="empty_trash">Empty trash</string>
+    <string name="empty_trash">Empty Trash</string>
 
     <!-- Conversation list screen overflow menu in spam folder [CHAR LIMIT=50]  -->
-    <string name="empty_spam">Empty spam</string>
+    <string name="empty_spam">Empty Spam</string>
+
+    <!-- Dialog title for empty trash confirmation dialog [CHAR LIMIT=100] -->
+    <string name="empty_trash_dialog_title">Empty Trash?</string>
+
+    <!-- Dialog title for empty spam confirmation dialog [CHAR LIMIT=100] -->
+    <string name="empty_spam_dialog_title">Empty Spam?</string>
+
+    <!-- Dialog message for empty folder confirmation dialog [CHAR LIMIT=300] -->
+    <plurals name="empty_folder_dialog_message">
+        <item quantity="one">This <xliff:g id="count">%1$d</xliff:g> message will be permanently deleted.</item>
+        <item quantity="other">These <xliff:g id="count">%1$d</xliff:g> messages will be permanently deleted.</item>
+    </plurals>
 
     <!-- Strings used for accessibility for the items that toggles the drawer action  -->
     <string name="drawer_open">Open navigation drawer</string>