[adb] Document and harden async transport flow. Add detailed documentation to the global 'transports' map and its associated helper functions in adbd auth. These helpers are used to safely pass 'atransport' objects across asynchronous boundaries (like user authorization popups or background TLS handshakes) without risking Use-After-Free. Also add CHECK_LOOPER_THREAD() assertions to ensure these functions are only called on the main fdevent thread, which is a requirement for the thread-safety of the global map and the underlying weak_ptr. Finally, refactor transport_from_callback_arg to use std::map::extract, which simplifies the code and ensures the entry is always removed from the map upon retrieval. Bug: 499053500 Test: atest adb_test --host Flag: EXEMPT BUGFIX Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:ca8689b1047a05598bb42ff034652ea679dd6056 Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:ea885606faf989081914b3aa699d42d0a46abb28 Merged-In: Ic419a1fb6f0df1ff868a1a31a33ded8a0ae7b2e8 Change-Id: Ic419a1fb6f0df1ff868a1a31a33ded8a0ae7b2e8