Fix whenToCheck returned by AttentionDetector

There could be a situation in which updateUserActivity would not be
called within [dim_t-3s(attention_timeout), dim_t], in which case we
will miss the "bad" event. Fix that by returning the computed
whenToCheck value, which is nextScreenDimming-attention_timeout.

Test: manually with logging
Bug: 111939367
Change-Id: I66e4a1e86582afe06a277ba8dbb127583940ea19