Split NotificationGutsManager out of StatusBar.

This CL introduces NotificationDisplayer, which is an interface
abstracting the notification displaying related functionality of
StatusBar. This is the first CL in a series of refactoring CLs breaking
up StatusBar into re-usable components. In order to keep the code
reviews a reasonable size I've had to leave a few TODOs. For example,
later I plan to pull out an interface from NotificationStackScrollLayout,
which NotificationGutsManager (and others) would use. But, then
everything would end up being pulled into the same CL so for now it's a
TODO.

Bug: 63874929
Bug: 62602530
Test: runtest systemui
Test: Compile and run
Change-Id: I482a0554aa13aefce4c48807a627cd5ecd820880