Change shadow drawing for bubble bar icons.

Removed an outline provider for the BubbleView, so framework shadows are
not drawn. Use BubbleBarBubbleIconsFactory to generate icon with the
shadow so it looks consistent with the taskbar and the hotseat bar.

Fixes: 345490679
Flag: com.android.wm.shell.enable_bubble_bar
Test: manual, screenshot before fix:
https://screenshot.googleplex.com/86ZhQENANboZK87
screenshot after fix:
https://screenshot.googleplex.com/3hNboaSaS9tZ9DS

Change-Id: I9ae3624e652e4f609b29ae0f57a47d4a6d01e94b