gnirehtet v2.4

Gnirehtet v2.4

Changes since v2.3:

• Require FOREGROUND_SERVICE permission (necessary for Android 9)
• Use an activity (instead of a broadcast receiver) as entry point,
  so that the intent is always allowed by Android (#112, #117, #146,
  #150, #151, #161, #178, #180, #187, #190, #199, #227, #242)
• Fix cleanup race condition in Java relay server (#154)
• Workaround rust/mio issue on MacOS (#136)
• Add command-line option to use a custom port (#210)