Refactor ToastPresenter to perform show()/hide()

In order to support multi-user, we need to create a new context based on
the user id and retrieve the services from it
(http://b/151414297#comment9). This meant retrieving the services in
ToastUI.showToast() instead of on its constructor, which would make the
code diverge from Toast$TN.handleShow(). In order to avoid that, now
seemed a good time to refactor ToastPresenter to perform show() and
hide().

This means ToastPresenter will now be instantiated in every request for
a new toast in ToastUI, but fortunately with the refactor we were able
to get rid of ToastEntry (which was also beign instantiated in every
request).

Also found out a bug with this where window tokens were being used to
locate the toasts instead of the (non-window) tokens. This is a bit
confusing because the method NM.finishToken(package, token) receives a
non-window token to locate the ToastRecord and then finish its window
token. This didn't have any side-effects because NM itself finishes the
tokens after a time-out. Added a test for this.

Bug: 152973950
Test: atest android.widget.cts29.ToastTest android.widget.cts.ToastTest
      ToastWindowTest ToastUITest NotificationManagerServiceTest
      LegacyToastTest
Change-Id: I13cf18890ca22022adb7576c8ecf3285a9b82299