Don't crash or wtf if the interface doesn't exist.

This prevents the code from issuing a Log.wtf due to calling
transitionTo in ClearingIpAddressesState#enter if the interface
does not exist when IpClient starts provisioning is called.

This prevents IpClientIntegrationTest from crashing on eng builds
due to the Log.wtf being unhandled.

Bug: 155005801
Bug: 152723363
Test: atest NetworkStackNextIntegrationTests:IpClientIntegrationTest#testRestoreInitialInterfaceMtu_NotFoundInterfaceWhenStartingProvisioning
Original-Change: http://aosp/1290575
Merged-In: I4f34343816202c996f9227b72d453ea4a464f4b8
Change-Id: I4f34343816202c996f9227b72d453ea4a464f4b8