Show key preview instantaneously

This change will display the key board preview instantaneously when
the key is pressed.  Dismissing the key preview will be delayed
slightly after the key is released, as we do now.

Because in multi touch mode, we should instantaneously change keyboard
layout when modifier key, such as shit and symbol, is pressed.

Change-Id: I49348d563ca0f40f5a0a236bdcd0bb07eedd599b