Refactor SystemUI so the status bar isn't a Service of its own.

There is now one SystemUIService, which starts the status bar service.
Pretty soon there will be other things running in here too.  This way
we don't need to have each of them started by something individually.

This also moves the choice between tablet and phone status bar into
SystemUI.apk, which seems like a much better place for it.

Change-Id: Ib69ef2f43d648764f8dbb52008f5d036a1ee07d9