Improve logic for when to display instant result

Bug: 21497671

Fix mChangedValue handling, so that it is only reset after an actual
evaluation, and is set when an expression is "collapsed".

Consistently produce instant results for solitary pre-evaluated
expressions if and only if they involve an abbreviation.

Change-Id: I4e1f824e2353cbe78b1827f3930c72666832cff4