Hide the IME when dragging the expanded view for bubble bar

Otherwise dismiss target isn't available and there's a weird inset
on the bubble bar expanded view.

Flag: com.android.wm.shell.enable_bubble_bar
Test: manual - expand bubble in bubble bar, show IME, start dragging
               the view
             => observe that IME is dismissed
Fixes: 359390685
Change-Id: I56a1cf168fb06af71f7c6752f5642640e1768fc8