API refactor: context.startForegroundService()

Rather than require an a-priori Notification be supplied in order to
start a service directly into the foreground state, we adopt a two-stage
compound operation for undertaking ongoing service work even from a
background execution state.  Context#startForegroundService() is not
subject to background restrictions, with the requirement that the
service formally enter the foreground state via startForeground() within
5 seconds.  If the service does not do so, it is stopped by the OS and
the app is blamed with a service ANR.

We also introduce a new flavor of PendingIntent that starts a service
into this two-stage "promises to call startForeground()" sequence, so
that deferred and second-party launches can take advantage of it.

Bug 36130212
Test: CTS

Change-Id: I96d6b23fcfc27d8fa606827b7d48a093611b2345