Fix issue #34471029: Don't allow audio use from background apps.

This is becoming a common pattern (keeping track of which processes
are cached and not allowing them to do stuff in that state), so I
am turning this in to a general mechanism for monitoring this state
through the activity manager's IUidObserver.  Now we can just have
AudoService implement its own IUidObserver to get this state and
update which uids it is blocking.

This required making some changes to uid change reports so that
the integer is now a bit mask instead of an enumerations, but that
is what it was already turning in to anyway.  (This gets rid of
the crazy GONE_IDLE constant that we'd needed to add before because
it wasn't a bit mask).

Eventually the power manager should be changed to be told about
these changes to cached state instead of listening to every proc
state change, but we'll do that later, it is more disruption than
I want to take for now.  However, while working on this, I noticed
that the power manager had regressed in the cached uids it would
actually block, because the activity manager was no longer telling
it about all uids that are idle.  (I think this happened when I had
to change the default idle state of UidRecord to true.)  So I am
adding a bit of new code to keep track of what idle state we last
reported to observers, to make sure we tell it about newly created
uids that are idle but have never actually become active.

Test: runtest -c com.android.server.am.ActivityManagerServiceTest frameworks-services

Change-Id: I7bfd46bacadd4cab2a69f40e6e52afb4e67b456a