Joining the thread before the MtpFfsHandle object's destruction

We're detaching the thread t in sendEvent. This means the thread
continues to execute independently, even after the MtpFfsHandle object
might have been destroyed. Instead of detaching threads, Join the thread
before MtpFfsHandle object's destruction.
Bug: 355382936
Flag: EXEMPT bug fix
Test: Build mtp_handle_fuzzer and run on the target device
Test: Run mtp in host and device mode

Change-Id: I924704575af89e747793c5d8647570d31cd14af9