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/keystore_cli_v2.cpp b/keystore/keystore_cli_v2.cpp
index a14800b..777db33 100644
--- a/keystore/keystore_cli_v2.cpp
+++ b/keystore/keystore_cli_v2.cpp
@@ -630,8 +630,7 @@
     CommandLine* command_line = CommandLine::ForCurrentProcess();
     CommandLine::StringVector args = command_line->GetArgs();
 
-    std::thread thread_pool([] { android::IPCThreadState::self()->joinThreadPool(false); });
-    thread_pool.detach();
+    android::ProcessState::self()->startThreadPool();
 
     if (args.empty()) {
         PrintUsageAndExit();