Added an onClick listener to the bubble bar.

Previously, clicks were not handled because no listener was set on the
bubble bar.

Fixes: 383622825
Test: Manual. Ensure bubbles are on the home screen and set the
navigation mode to 3-button navigation. Click on the collapsed bubble
bar and observe the bar expanding.
Flag: com.android.wm.shell.enable_bubble_bar

Change-Id: I99e1c20e40a9dd9da97b68817734c8ca1f656baa