Close ResearchLogger upon onFinishInputView

Previously, ResearchLogger#stop() was called both in
LatinIME#onFinishInputView() and in LatinIME#onWindowHidden().  This
resulted in multiple logs being written.

Since onFinishInputView is the more reliable of the two (it is called
in InputMethodService#onDestroy; onWindowHidden is not), the code now
uses onFinishInputView as a stopping signal.

Change-Id: Iae4b8c3bdab226027624eeab19b3737367e4a108