Add a setTeardownDelayMs API to NetworkAgent.

This allows transports to request that when the network is
disconnected, the system should delay destroying the native
network until the specified time has passed after the network
disconnected.

Bug: 181941583
Test: next CL in the stack
Change-Id: I9765f1c9d1e55c23c6d583d6709dbe06505975b1