Don't send the same values to onUpdateSelection repeatedly

If the IME is repeatedly changing the text in its
onUpdateSelection handler, this will crash it with a
stack overflow exception. It's better than the old behavior,
which would result in a busyloop likely to make the
device completely unresponsive.

Bug: 10301239
Change-Id: I170cfb8ef20fc056d4725931890a987aefcaea8b