Delay releasing validation network request to ensure smooth switch.

Upon data switch when a network is validated, as PhoneSwitcher confirms
the switch, Validator shouldn't release the network request immediately.
Otherwise, it may result in disconnecting the new network first, followed
with bringing it up again. So when releaseAfterValidation is true, we
delay releasing the request by 500 ms.

Bug: 140070796
Test: unittest
Change-Id: Ib24ad282e7caadf91a7b8432885bd28022e5822a
Merged-In: Ib24ad282e7caadf91a7b8432885bd28022e5822a