Properly get timestamp position in DiscussionParser

The code was looking at the [0] element for the matched position
of timestamps, while preg_match returns it in the [1] element.

Bug: 53132
Change-Id: Ibfd3f2b86b007f28f73a137defb80276fb830d28
Follows-Up: I6c636b055bcd25760aee848aea71fe4044c7e1be
Merge "Properly get timestamp position in DiscussionParser"