fixing RealtimeSleepTimer to work in concurrent threads

[theBoatman]

This pull request is the reincarnation of signalapp/libsignal-service-java#70 because @alan-signal requested the following changes:

• Move the class RealtimeSleepTimer from from libsignal-service-java to Signal-Android
• rename it to AlarmSleepTimer

For any other info on this change please read original request, and especially alans post.

I provided a test build if anyone wants to try it out.

If there is any problem or the need for further changes feel free to ask.

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[alan-signal]

Thanks, please sign our Contributor License Agreement.

[ghost]

@theBoatman thank you very very much for the patch. Maybe now I can finally use the official release! ❤️

[greyson-signal]

@theBoatman Thank you! I'm well aware that it took us a (very) long time to get to this, so thank you for sticking with it ❤️

[theBoatman]

@greyson-signal I am glad that I could help and happy that it is finally done.

My final question on this topic is: Are you and @alan-signal interested in further patches for the gappless code? I am sure that there are more problems or things that could be better. (I have at least two minor things in mind to take a closer look at.) Does it make sense to make further pull requests even if the addressed problems are not as important as this one here?