Prevent the potential NPE when toggling interface quickly.

Querying interface parameters in different states has race if
toggling interface very quickly, for example, remove the interface
before transition to ClearingIpAddressState and add it back soon
immediately, that will introduce the different interface parameters
value. Global member "mInterfaceParams" is null when query occurs
in the ClearingIpAddressState since the interface has been removed
while local variable "params" isn't, then NPE will be thrown upon
retrieving the interface index when attempting to restore MTU.

Bug: 162808916
Test: atest NetworkStackIntegrationTests --iterations 100
Change-Id: I346f917aa1ff09342940d3a9a0c468483c279cdb