Backport system default callback to Q

To allow tests using the shim to pass on Q, backport the shim to Q in
addition to R. In particular,
ConnectivityManagerTest#testGetMultipathPreference uses
registerSystemDefaultNetworkCallback to verify whether the default
network was set to metered/non-metered successfully.

Bug: 182961265
Test: atest ConnectivityManagerTest#testGetMultipathPreference
Change-Id: I58cd0be7dd01f03625639393042436d2b47d99c0