No notifications on fdroid flavour when phone enters 'sleep' mode (> android 6.0)

[BillCarsonFr]

Step to reproduce:

• ensure that the app is not white listed from battery optimization
• Close riot app
• Wait until the phone enters doze mode
  The listening for event notif is displayed on the system tray but the phone will not receive any notifications.

• additional
  => use 'adb shell dumpsys deviceidle force-idle' to force the doze mode.

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Expected: in f-droid mode Riot should ask user to whitelist the application from battery optimization (not the case currently).