Ignore public volume state change event of removed user in Automotive

Volume state change event may come from removed user, in this case,
mountedUserId will equal to UserHandle.USER_NULL (-10000) which will do
nothing when call cancelAsUser(), but cauase crash when call
notifyAsUser(). Here we return directly for USER_NULL, and leave all
notifications belong to removed user to NotificationmanagerService, the
latter will remove all notifications of the removed user when handles
user stopped broadcast.

Bug: 156667908
Test: manual - remove user while usb plugged in
Change-Id: I23670e070a17b6da8bdd9eb336aa053e3b799b16