Clarify thread safety of BinderTransport and its TransactionHandler implementation

[shivaspeaks]

In PR #11981, a potential data race on the acknowledgedIncomingBytes field in BinderTransport was identified. However, it was noted that all incoming Binder transactions are FLAG_ONEWAY, ensuring that only one thread from the Binder thread pool invokes handleTransaction() at a time.​

Despite this, the BinderTransport class is annotated with @threadsafe, which may be misleading since it implements TransactionHandler directly. To address this, BinderTransport shall delegate TransactionHandler to a private inner class not marked as @threadsafe. This change would clarify the thread safety guarantees and prevent potential misunderstandings.​

Action Items:

• Refactor BinderTransport to delegate TransactionHandler to a private inner class.
• Update documentation and annotations to accurately reflect thread safety guarantees.
• Ensure that the refactoring maintains existing functionality and passes all tests.​

[ejona86]

CC @jdcormie

[jdcormie]

Looks like handleTransaction() is called directly by at least one unit test. If that method becomes inaccessible from outside BinderTransport, you'll have to find some other way to cover that test case.