Merge "Rename btu_task.h -> main_thread.h" into main am: 6a1ff548 am: 6f67790d am: 4932916f

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Bluetooth/+/2786431



Change-Id: I20c06cbb53e19f716ba74a3de709d4aba63f1dd4
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>