Use AsyncTask.SERIAL_EXECUTOR as the default executor for scan

Because there will be multiple callbacks for each scan and they must be
invoked with the same order as they are received by the platform, we
should use AsyncTask.SERIAL_EXECUTOR instead of
AsyncTask.THREAD_POOL_EXECUTOR as the default executor.

Bug:74840070
Test: Unit Test, CTS Test
Change-Id: I7333940dad38f7e400a4124486bdb21cea0d5220