Make sure the ime height is set to 0 when the ime disappears

Previously we were setting the offset always which means the bubble
position wasn't quite where it should be.

Test: manual - have stack of bubbles, note how far to bottom of screen
               the stack can be placed
             - expand them
             - focus the keyboard
             - collapse the stack
             => note how far to bottom of screen the stack can be placed,
                should be same as step 1

Change-Id: I0e3d788a2edcd42b56ce8ee4e350be3ddf89183e