Allow NETWORK_LOST disconnections to retry

This change adds the ability for disconnection requests to specify
whether the full VcnGatewayConnection should be torn down. This is
useful in cases where the cause is an ephemeral failure, and can be
restarted at some later point (eg an underlying network appears, or a
new NetworkRequest is filed that brings the VCN up.)

Bug: 179944275
Test: atest FrameworksVcnTests
Change-Id: Id1813bcbbb80541be97cc383ea9d1fe08554b7ae