Fix meizu deadlock

Some devices (Meizu) assume that the video encoding thread has a
Looper. By moving video encoding to a separate thread, commit
feab87053abcceded41342d9d856763dedc09187 broke this assumption.

Call Looper.prepare() from this thread to fix the problem.

Fixes #4143 <https://github.com/Genymobile/scrcpy/issues/4143>