Update Android app to handle API 26, or migrate to target API 28

[str4d]

Google Play currently requires that app updates target at least API 26, and will bump this to API 28 from November 1st 2019.

The Bote Android app (which was last published in 2015) does not meet this requirement:

https://github.com/i2p/i2p.i2p-bote/blob/9019f4025282262a6bbdc4ff98c4c0db64e3c8a0/android/build.gradle#L9

There are four major API changes we need to handle:

• [ ] API 23: Runtime Permissions
• [ ] API 24:
  • [ ] Doze and App Standby
  • [ ] Permissions changes
  • [ ] Linking changes
• [ ] API 26:
  • [ ] Background Execution Limits
  • [ ] Notification Channels
• [ ] Api 28:
  • [ ] Power Management
  • [ ] Foreground service permission
  • [ ] Privacy changes

This issue is for tracking progress of the migration.

[str4d]

118 bumped targetSdkVersion to 26 without investigating any of the above changes. This issue is now also a meta-issue for any bugs that arise as a result. Close condition remains that the above changes need to be handled.