Refactoring SpannableStringBuilder

Made TextWatcher notification process clearer by moving it to
a single place, with methods renamed.

Also reverts CL 177544: we cannot broadcast span chages just yet,
the layout has not been reflown. A future CL will change this
behavior to make sure span changes are correctly broadcasted.

Change-Id: I9ef88dce91dff5f5f45e2845d5b3f18f1c853de3