netd_integration_test:BinderTest#GetFwmarkForNetwork - save/restore default network

Otherwise network connectivity after the test is run ceases.

Tested: via atest
Bug: 157114644
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I3e1dc027670644c1b4f0b2aa35a543c398b103be