Loader screen delay during firmware installation over BLE

[Hannsek]

During firmware installation via BLE, the loader screen on the device takes long time (approx. 20 seconds) to appear, even though Trezor Suite already shows installation progress (e.g., 12%).
This discrepancy creates confusion and gives the impression that the device is unresponsive.

[mmilata]

The code that does this can be traced somewhere to 2018: 6e81a68.

I guess it tries to work around the fact that there are actions done after the first chunk (validation, flash erase). So it avoids seeming stuck at the first chunk for a while, which probably does not work that well with T3W1 fw size.