Adding doze mode observation

[aperfilyev]

#115 this is how I see it.

According to https://developer.android.com/training/monitoring-device-state/doze-standby.html#restrictions, network access is suspended in Doze mode.

[codecov-io]

Current coverage is 72.92% (diff: 50.00%)

Merging #117 into master will decrease coverage by 4.09%

@@             master       #117   diff @@
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  Files             7          7          
  Lines           161        181    +20   
  Methods           0          0          
  Messages          0          0          
  Branches         17         22     +5   
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+ Hits            124        132     +8   
- Misses           26         36    +10   
- Partials         11         13     +2   

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[pwittchen]

Thank you for this PR! I'll review and test it as soon as I can.

[Kisty]

Can you rebase the PR on the master branch and push changes to your pr branch? I think the code coverage tool fails have been fixed with a recent commit.

[pwittchen]

@aperfilyev Can you squash your commits into one commit? You can use this website: http://rebaseandsqua.sh/.

@Kisty codecov.io fails the PR because of decreased code coverage. Decreasing code coverage by 4% is not a big deal here and we can increase it later. I'm gonna add more "liberal" configuration for codecov.io, so it won't fail the PR if the drop of code coverage was very small.

[pwittchen]

After looking at the code and this PR, I think that creating a new strategy like MarshmallowNetworkObservingStrategy would be a good idea in the case of Doze mode. I haven't noticed that during the discussion in issue #115 and I wasn't aware of the fact that this feature is available since Android M. Nevertheless when you squash your commits, I can merge it and we can refactor it later quite easily, so finally we'll have 3 strategies for network monitoring.

[aperfilyev]

@pwittchen done.