binder_rs: Make ParcelableHolder thread-safe

Replace a few non-thread-safe types and traits in
ParcelableHolder with their thread-safe equivalents,
i.e., RefCell => Mutex, Rc => Arc, Downcast => DowncastSync.

Test: atest aidl_integration_test
Change-Id: If0ba81232b482e6427ec3e62d2b4474615a69147