Remove FLAG_FOREGROUND_SERVICE on Service.stopForeground()

When services call Service.stopForeground(), remove
FLAG_FOREGROUND_SERVICE from the notification that was supplied
to Service.startForeground().

This enables services to post notifications that become user
dismissable when they switch to being a background service.

Restrict this to targetSdk=L apps to reduce the risk of breaking
existing apps.

Bug: 17551106
Change-Id: Iff8541e5bb2a23ad1fbc9ad80df5fd6eb683148b