Improve bigram probability computation for decaying dicts.

Without personalization:
Total words: 1079345, Success Num: 819749, Success Percentage: 75.949%
Bad Failures, with auto-correction (typed word == expected word, output word != expected word): 1754, Bad Failure Percentage: 0.163%
Failures, with auto-correction (F-C): 28463, F-C Percentage: 2.637%
Max Keystrokes: 6074285, Min Keystrokes: 4649326, Keystroke Saving Percentage:23.459%

With current probability computing logic:
Total words: 1079382, Success Num: 838329, Success Percentage: 77.667%
Bad Failures, with auto-correction (typed word == expected word, output word != expected word): 1332, Bad Failure Percentage: 0.123%
Failures, with auto-correction (F-C): 28558, F-C Percentage: 2.646%
Max Keystrokes: 6074503, Min Keystrokes: 4474102, Keystroke Saving Percentage:26.346%
Remove isof files.

With new probability computing logic:
Total words: 1079356, Success Num: 844954, Success Percentage: 78.283%
Bad Failures, with auto-correction (typed word == expected word, output word != expected word): 1306, Bad Failure Percentage: 0.121%
Failures, with auto-correction (F-C): 27214, F-C Percentage: 2.521%
Max Keystrokes: 6074477, Min Keystrokes: 4243021, Keystroke Saving Percentage:30.150%
Remove isof files.

Bug: 16547409
Change-Id: I3d2a49c7aaa2c0f6835c52ef72d22466ee225789