bug: 6584677

While looking at bug 6584677, we found that LatinIME always spawns an unnecessary thread
for Usability Study Mode.  We don't assume this change will address the root cause of the
bug, but we'd like to remove any suspicious noises in bugreports.

Change-Id: Ie47d104f6cd77ec2ad164f13b4212cb4a70bcf1d