SuggestionSpans are removed around edited text.

SuggestionSpans do not make sense anymore when the text they
refer to is modified. Removed these at the lowest possible common
level: In the SpannableStringBuilder that is used to back the Editable.

This way, IME do not have to care about removing these when they change
text. And they cannot forget to so either.

Also fixed a bug in TextView's paste with multi-item paste text (never
exercised, since we have no source for such a thing).

Change-Id: I08ed921f8c04ffb1a00936a3e554a85ee82f103c