Don't send the same PendingIntent more than once.

Fixing a bug where a NetworkRequest's PendingIntent can be sent more
than once when networks are rematched before the intent completes.

Added a small delay before removing the request to give the receiving
client an opportunity to put in its own request. The delay value is
configurable via Settings.Secure.

Bug: 18614074
Change-Id: Iac7c5e5a04f42f2b6794e9e22349cc631bebeab7