Explicitly clears signal upon OnProducerGained in consumer channels

And fix some unreadable code around buffer state
and a wrong-but-no-effect-yet fence return.

Bug: 120569702
Test: patch ag/4833259 and not see washing machine
Test: AHardwareBufferTest BufferHubBuffer_test
buffer_hub_queue_producer-test buffer_hub-test buffer_hub_queue-test
dvr_buffer_queue-test dvr_api-test libdvrtracking-test

Change-Id: I63b18a78ea05d2084b2ed29fbcaacbb652d48846