Show infinity for large notification counts

This patch adds a new string to SystemUI for displaying the infinity
symbol when a notification has a count larger than 999.  This only
affects the notification icon in the status bar and does not change
the text displayed in the actual notification.

These changes were manually cherry-picked from cm-13.0.

Change-Id: I3481aac13cd90ee20ed48194df3ec789c16c8b4b