Use more tags to help the ICU detector.

The detector only gave non-ascii data to ICU. In some cases that could
result in very short data, for which ICU would issue a low confidence
level for the actual encoding. By padding the data with additional
(ascii) tags, we improve accuracy for such files. Becauses this can
reduce accuracy in other cases, only do this when the initial confidence
is low.

b/13473604

Change-Id: I63d932043155c310b0e358cdf2d37787961e94b7