pw_bluetooth_sapphire: Refactor TxEngine to allow queueing SDUs

This change allows SDUs to be queued in the channel itself rather than
maintaining an internal queue in each engine. This is currently only a
refactor and should NOT change any actual behavior.

The change can be summarized as:
  * `TxEngine::TxChannel::SendFrame` is now used instead of callbacks.
  * `TxEngine::TxChannel::GetNextQueuedSdu` has been added to allow
    TxEngines to retrieve SDUs from the channel queue.
  * `TxEngine::QueueSdu` has been removed entirely, instead the channel
    will notify the TxEngine that a new SDU is available for processing
    by calling `TxEngine::NotifySduQueued()`. The engine can then make a
    decision on whether it is ready to process the SDU immediately or
    wait until some other state changes for flow control or similar.

The enhanced retransmission mode does not (yet) use this queueing
functionality as of this CL, this is for two reasons, both of which are
intended to minimize the risk of this change breaking existing
functionality:

 1. The intent of this CL is to be a refactor only, so actual behavioral
    changes are considered out of scope for this change. Behavioral
    changes will be introduced in a future CL.
 2. The enhanced retransmission mode has some complexity that needs to
    be accounted for in a more detailed CL.

Test: fx test with all tests under src/connectivity/bluetooth
Test: Manual of A2DP source/sink and AVRCP.
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/988215
GitOrigin-RevId: 6760ecebd67139e37d0b207ca5c70fa8583f6059
Change-Id: Ibb0af949ce75ef26c92275ec76329186f93e6f47
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/218915
Reviewed-by: Faraaz Sareshwala <fsareshwala@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Jason Graffius <jgraff@google.com>
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