Hide IME on touching down on a Bubble.

This fixes the issue where the dragged-out bubble and the dismiss target rendered under the IME. It also improves animations all-around since it starts the IME animation earlier (vs waiting for ACTION_UP).

While the IME does hide earlier, once your finger is on a Bubble, there is nothing you can do that would not hide the IME anyway - dragging it out hides it, and if you lift your finger, you'll either switch bubbles or collapse the stack, both of which hide the IME.

Test: have IME, drag out bubble
Fixes: 158861545
Change-Id: I903897eca3c516bc953cbd70ab2102bbc4a81a4f