Add feat 'run in background'

[marckassay]

This feature should be enabled by default in the Settings view. Below are relevant links:

• https://ionicframework.com/docs/native/background-mode
• https://github.com/katzer/cordova-plugin-background-mode

[marckassay]

The plugin listed above has breaking limitations. One of these issues is this one.

Having AIT as a 'foreground service' seems to a better solution using this plugin. Along with this plugin for Notifications.

Although, Beacons may seem ideal, I think that since this is a timer app, it might be pushing the limitations for them to scan every 1.1 seconds.

We recommend not reducing the scan period to be less than 1.1 seconds

• ref

ref: katzer/cordova-plugin-background-mode#400 (comment)