[ledger] terminate CloudProvider after each validation test

Validation tests may create a new cloud provider process for each test.
We proxy the requests on the FIDL channel so we can know when the client
closes the channel and terminate the cloud provider instance launched
for this test.

TEST=run validation tests, fx shell ps, no cloud provider is running

Change-Id: I522901d2b913d06b8ab207b14ddd116f32394863