fix keepAlive interval for non GCM Android #6644

[rkohrt]

This is a rewrite of #7100 with the suggested changes of Moxie. It should achieve the same thing as the original pull request while not braking the API.

I added an AlarmManager alarm, to periodically send a KeepAlive message and waking any threads that may be stuck in blocking wait, while the device is sleeping.

This fixes #6644
It depends on my changes in signalapp/libsignal-service-java#48

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[moxie0]

The way I want to do this is as follows:

1. In the Android app, the logic does not change at all. The diff to the Android app should be 1 line.

2. In the service library, there are two different implementations of a single sleep interface. One, the default, uses Thread.sleep(). The other, an optional implementation for non-play devices, uses the alarm manager.

3. All together, the logic and implementation for Play devices will be exactly the same as it is now

Thanks!.

[theBoatman]

@moxie0 I have an implementation that I think could match your wishes. Can you please take a short look whether this goes in the right direction?

https://github.com/theBoatman/libsignal-service-java
https://github.com/theBoatman/Signal-Android

I was not able to put the android implementation of sleep in the service library. (It compiled, but I was not able to use it in the app.) That is probably just a dependency issue and not a big blocker. The key of this is that the implementation of AndroidThread.sleep() works and matches your needs and I think that might be the case.

The code works for me at the moment, but needs a little bit more testing. If you think that version has a chance to get merged, then I will continue to work on it. (Testing, coding styles etc...)