Merge branch '47-sign-in-reminder' into 'master'

briar-android/artwork/notification_ongoing.svg

briar-android/artwork/notification_reminder.svg

briar-android/src/main/AndroidManifest.xml

...java/org/briarproject/briar/android/AndroidComponent.java

...project/briar/android/AndroidNotificationManagerImpl.java

...ain/java/org/briarproject/briar/android/BootReceiver.java

...ain/java/org/briarproject/briar/android/BriarService.java

...java/org/briarproject/briar/android/TestingConstants.java

...project/briar/api/android/AndroidNotificationManager.java

...r-android/src/main/res/drawable/notification_reminder.xml

briar-android/src/main/res/values/strings.xml