Tell the factory it is already serving a request.

This is a cherry-pick of ag/607226 that has been rebased on
top of four years of changes and with comments addressed.

Gives each factory a serial number and propagates it to every
NetworkAgent so when a score comes back indicating a request is
being handled the factory can account for it properly.

Without this, a new request that's already handled by a network
offered by a factory will not cause an increment of the factorys
ref count. Concretely this results in issues like the RAT icon
not being displayed in spite of the network actually being up
and usable.

This will be ported to AOSP as soon as possible, but immediately
some master-only WiFi tests need to be adjusted with this change
which would not let me submit to AOSP.

Bug: 18637384
Bug: 29030667
Test: manual
Test: atest frameworks/opt/telephony/tests/telephonytests
Test: atest frameworks-net
Test: atest CtsNetTestCases CtsHostsideNetworkTests
Change-Id: I597ac588f76dd507512ff02868fd1310b7e63f7e
Merged-In: I597ac588f76dd507512ff02868fd1310b7e63f7e