Implement background vs foreground broadcasts

Before now, receiving a broadcast would cause a process to be hoisted
to foreground priority / cgroup.  This is no longer the case: broadcasts
by default are handled in the background, with a suitably increased
timeout interval.  When a given broadcast needs to be dealt with in a
more timely manner, the issuer can set the new FLAG_BROADCAST_FOREGROUND
flag on the Intent, which will produce the old foreground-priority
behavior.

To avoid priority inversions, foreground broadcasts are tracked on a
separate outgoing queue and can be in flight simultaneously with a
background-priority broadcast.  If there is already a background-level
broadcast in flight to a given app and then a foreground-level one is
dispatched to that app, the app [and its handling of both broadcasts]
will be properly hoisted to foreground priority.

This change is also essentially the first step towards refactoring the
broadcast-handling portions of the Activity Manager into a more
independent existence.  Making BroadcastQueue a top-level class and
regularizing its operation viz the primary Activity Manager operation
is the next step.

Change-Id: If1be33156dc22dcce318edbb5846b08df8e7bed5