Target Android API level 27

[daniele-athome]

This is a big one. And I have time until 2018-11-01.
The main part is #1198. Technical notes are there.

---

https://developer.android.com/about/versions/oreo/android-8.0-changes

[codecov]

Codecov Report

Merging #1228 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #1228      +/-   ##
=========================================
- Coverage    0.52%   0.52%   -0.01%     
=========================================
  Files         287     288       +1     
  Lines       25244   25290      +46     
  Branches     3305    3316      +11     
=========================================
  Hits          133     133              
- Misses      25098   25144      +46     
  Partials       13      13

Impacted Files	Coverage Δ
...java/org/kontalk/ui/prefs/MaintenanceFragment.java	0% <0%> (ø)	⬆️
...va/org/kontalk/service/gcm/GcmListenerService.java	0% <0%> (ø)	⬆️
...main/java/org/kontalk/service/DownloadService.java	0% <0%> (ø)	⬆️
...pp/src/main/java/org/kontalk/util/Preferences.java	0% <0%> (ø)	⬆️
...ntalk/service/msgcenter/StartMessageCenterJob.java	0% <0%> (ø)
...java/org/kontalk/service/NetworkStateReceiver.java	0% <0%> (ø)	⬆️
...a/org/kontalk/provider/MessagesProviderClient.java	0% <0%> (ø)	⬆️
...in/java/org/kontalk/ui/ComposeMessageFragment.java	0% <0%> (ø)	⬆️
...ontalk/service/msgcenter/MessageCenterService.java	0% <0%> (ø)	⬆️
...pp/src/main/java/org/kontalk/util/SystemUtils.java	2.73% <0%> (-0.1%)	⬇️
... and 5 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9a45e65...131be34. Read the comment docs.