Moving PanelViewController and PanelView to android.systemui.shade.

As PanelViewController seems very related to
NotificationPanelViewController it might be better for them to live in
the same package. Imports have been updated and the scoping has been
expanded as necessary. This is a follow up from ag/19202451.

Bug: 238333941
Test: atest SystemUITests
Change-Id: Ief8ad6dc74056ab3292b94c191eea0e5d209996c