Start IME animation if its insets update after show

Some IME might update its insets after showInsets callback, it
should abnormal but we still should be robust to handle such case.

Fix it by re-calling startAnimation. If it call while animation,
it should cancel previous one and start new one. If animation end,
it will run a new animation.

Bug: 151628536
Test: force stop Gboard, enter split mode and click text filed on
      secondary split.
Change-Id: I31d8e800ef9fc67406cd7c7902b1c597e9c8dcbe