Initial refactoring to move updateSelection to a better place

This is step 1 of a four-step change which will move where
updateSelection is called in regular text fields. The new
behavior will be to call it for each actual batch of
changes, instead of doing it in the render loop which leads
to both false positives and true negatives.

Bug: 8000119

Change-Id: I17bd91a129b18d5451fe1d8e7794ae20165de309