Ensure notification visibility on stopForeground(false)

If an FGS notification has been deferred but the app needs it to remain
present after the service exits the FGS mode, make sure that it gets
shown.

Bug: 193665032
Test: atest CtsAppTestCases:android.app.cts.ServiceTest#testForegroundService_deferThenKeepNotification
Change-Id: I1e9c5f1b27dd670639bd5e02b7f79839cfb5362e