Move 'no widgets found' to right pane and add rounded edges to right pane.

Previously 'no widgets found' appeared on the left pane, but in large screen landscape we want it to appear on the right pane. Additionally, we added rounded edges to the right pane that turn into sharp edges when we scroll.

Fix: 268558448
Fix: 268557435
Test: Turn on LARGE_SCREEN_WIDGET_PICKER and on tablet landscape mode go to the widget picker and search for a widget that doesn't exist, verify that 'no widgets found' appears on the right pane. Also verify that right pane has rounded corner that turn into sharp corners as we scroll
Change-Id: I6c89c63941e3b0680836828d0c7849d789cbce61