Call clearCallingIdentity when calling registerNetworkProvider in TNS

The caller to create a TestNetworkService via
ConnectivityManager#startOrGetTestNetworkManager should have
passed the MANAGE_TEST_NETWORKS permission check and the caller
doesn't need to declare the permission to pass the permission
check of registerNetworkProvider. So call clearCallingIdentity
when calling registerNetworkProvider in TestNetworkService.

Bug: 181573283
Test: atest CtsNetTestCases:ConnectivityManagerTest#testRequestBackgroundNetwork
Change-Id: Ia28627dacf933d1937978ed9709b975c9a4660ce