Fixed critical bug EXT-6847 (Default configuration has Away Timeout set to "Never")

Reason: changeset 97654b5b6313 for EXT-4843 [NUX] Set default Preferences. Setting was reset to 0.

Fix: restored default value 300. Also updated setting's description with supported combobox values (0, 120, 300, 600, 1800)

Reviewed by Vadim at https://codereview.productengine.com/secondlife/r/234/

--HG--
branch : product-engine

indra/newview/app_settings/settings.xml

@@ -4,13 +4,14 @@
     <key>AFKTimeout</key>
     <map>
       <key>Comment</key>
-      <string>Time before automatically setting AFK (away from keyboard) mode (seconds, 0=never)</string>
+      <string>Time before automatically setting AFK (away from keyboard) mode (seconds, 0=never). 
+        Valid values are: 0, 120, 300, 600, 1800</string>
       <key>Persist</key>
       <integer>1</integer>
       <key>Type</key>
       <string>S32</string>
       <key>Value</key>
-      <real>0</real>
+      <real>300</real>
     </map>
     <key>AdvanceSnapshot</key>
     <map>