Cleanup of timeout handling and message

Bug: 21470513

Prevent timeout message from disappearing on rotation.
Have long timeout setting survive rotation.  Following Justin's
suggestion, associate it with a given expression and reset when
expression is cleared.

This does mean that when you rotate a device displaying an expensive-
-to-compute result, the device will initially display the formula for
several seconds, before it redisplays the result.  Previously you
had to reenable the long timeout.  Neither is 100% ideal.

Change-Id: Ibf8e151dd37ebadf1e86adee4718e8fa8f66b975