implement heartbeat feature like on ios

[biblicabeebli]

• need to test how the nonce uniqueness works on android.
• need to check how the app processes notifications without a survey in them.
• what happens on android when it receives a notification while the app is actually-for-real not running.
• iirc android has different notification structure (title and subtitle??), will need to check what that looks like.
• use this work to investigate android notification squashing.

[biblicabeebli]

SitRep:

• While we need some testing on app persistence, especially on newer Android versions, (which is among the reasons why we are building this feature) Android is not in the same dire situation as iOS was.
• This feature has been implemented as a way to send the app a simple notification message.
• (though I have to finish researching this) Android doesn't have the same "notification loop" as iOS. iOS receives a notification, displays it, but doesn't poke the app until the user opens the app. Android has the app receive the notification and do something, and then it is up to the app to display notification text.
  • TODO: research how the android push notification priority stuff works.
• When the app is force-stopped or paused notifications are not shown (pausing does not stop background service activity, it just seems to stop notifications).
• we still want the nonce.

Even if the Android app cannot use this feature the ability to display a notification about reopening the app it clearly has a lot of uses, among them implementing a push notification asking the participant to update the app with a link to the respective study server's download link.

[biblicabeebli]

I am marking this feature as done, further discussion including fundamental utility of this feature for Android is located at:

• https://github.com/onnela-lab/beiwe-backend/issues/375