Don't start on boot when app has been manually stopped before

Closes #87 (closed)

@earthlingIB Testing instructions:

Scenario 1 (restarting device starts mailbox on boot if mailbox was set up and started before reboot)

• Set up mailbox on a fresh device and pair it with briar.
• Restart device. Confirm that mailbox does start automatically after boot. Be patient, it can take 1-2 minutes for the mailbox to actually start.
• "App starts" in all cases here means that the notification with the mailbox logo does appear in the notification area, not that the app's activity gets launched.

Scenario 2 (restarting device does not start mailbox on boot if mailbox was set up but was stopped before reboot)

• Set up mailbox on a fresh device and pair it with briar or reuse paired setup from scenario 1 and start the mailbox app.
• Stop the mailbox app using the "Stop" button from the status screen.
• Restart device. Confirm that mailbox does not start automatically after boot. As it can take quite a while this needs to be observed for a couple of minutes. Good to take into consideration the amount of time it took for the mailbox app to start automatically in scenario 1.

Scenario 3 (restarting device starts mailbox on boot if mailbox setup has been started but not finished yet, i.e. setup has reached qr-code stage, but pairing not yet complete for example because just not scanned by briar yet)

• Reset data for mailbox app or uninstall and reinstall.
• Start mailbox app, proceed until qr code is visible.
• Restart device. Confirm that mailbox app does start up automatically after boot

Scenario 4 (restarting device does not start mailbox on boot if setup has been started but cancelled during qr-code stage)

• Reset data for mailbox app or uninstall and reinstall.
• Start mailbox app, proceed until qr code is visible.
• Cancel setup using the cancel button
• Restart device. Confirm that mailbox app does not start up automatically after boot