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
diff --git a/keystore-engine/keystore_backend_binder.h b/keystore-engine/keystore_backend_binder.h
index 1db90f7..4c828c5 100644
--- a/keystore-engine/keystore_backend_binder.h
+++ b/keystore-engine/keystore_backend_binder.h
@@ -27,7 +27,7 @@
 
 class KeystoreBackendBinder : public KeystoreBackend {
   public:
-    KeystoreBackendBinder() {}
+    KeystoreBackendBinder();
     virtual ~KeystoreBackendBinder() {}
     int32_t sign(const char *key_id, const uint8_t* in, size_t len,
                  uint8_t** reply, size_t* reply_len) override;