Run a main looper in the cleanup process

Since a main looper is explicitly run in the main process, the
initialization of workarounds no longer calls
Looper.prepareMainLooper(), leading to a crash:

    java.lang.RuntimeException: Can't create handler inside thread
    Thread[main,5,main] that has not called Looper.prepare()

As a result, --power-off-on-close was broken.

Refs 283326b2f6fa3fdaeecc181f69a3a4bcd429c06a
Fixes #6146 <https://github.com/Genymobile/scrcpy/issues/6146>