Finish mocking in finally clause

Failure of testIpClientServiceAvailableAfterPolling could cause tests in
the same process to fail because the mockito session is not finalized.
Use try / finally to make sure finishMocking is called.

Test: atest ModuleNetworkStackClientTest
Bug: 139314310
Change-Id: I95cc24e789c5af5f655c875314ed97f50121166c