Move text toast creation to system UI

In order to do that, NotificationManager now calls
StatusBar.showToast(), which is in SystemUI. StatusBar then calls a new
component, ToastUI, which is responsible for rendering the toast. The
code for rendering the toast was extracted from the Toast class, so it
should behave identically.

Also refactored the code a bit on NotificationManagerService, creating
two children of ToastRecord (one for custom, other for text toasts).

The change is gated in Toast class in this CL, but it's also gated on
the system server, so apps can't circumvent the background block.

Bug: 128611929
Bug: 144754526
Test: atest android.widget.cts.ToastTest
Merged-In: Id0021cdc6f72f41b55ff1c5a4f09ae7687586460
Change-Id: Id0021cdc6f72f41b55ff1c5a4f09ae7687586460