Restructure display formatting, use TextView for formula.

This fixes issues with inappropriate keyboard popups and cleans
up the code.

This substantially rewrites the result formatting code;
it intentionally removes a heuristic to avoid conventional scientific
notation for short results, which only seemed to confuse things.
There are some other tweaks and simplifications.  Transitions
to and from conventional scientific notation should be smoother,
though there is still an issue with numbers like 10^-30.
The formatting code is completely extracted from Evaluator, cutting
off various tentacles, and moved to CalculatorResult.

Fixes a bug that resulted in digits being inappropriately
displayed without an exponent if the decimal point was just off
the left edge of the screen.

It's still not clear that we should always be integrating the
exponent into the result window.

Bug: 20483801
Bug: 20480081

Change-Id: I1bb777b871e9fa8b6cd510c533e4b94bde6d4a20