Fix keystore_backend_binder

Since the keystore AIDL interface became asynchronous we need a thread to handle the
callbacks.

With this patch keystore_backend_binder starts a thread pool when a
backend is created.

Also change keystore_cli_v2 to use startThreadPool instead of starting a
binder thread explicitely.

Bug: 111443219
Change-Id: Ic5b19d95f51a24d823825d5874ec85eeabd9ef5f
3 files changed