#54 Add heartbeat notifications 💓

[lukeswitz]

Feature Description

Fills request from issue #54

• Adds option to send a beating heart emoji while the app is active
• Timer setting, minimum 1 minute and defaults to 5.
• Sent to the saved phone number via Signal as: 💓
• Used JNI compatible strings to avoid parsing errors on lower Android versions instead of adding EmojiCompat dependency

Opportunities:

• Could be tied into the MonitorService to send a 💔 broken heart on some exceptions

• Tested Android 5-8: Review/input appreciated, I don't think it needs a heading in settings, should be moved into Monitor, etc.

(Note: The first two commits are still in place but have since been changed to match current state. There to illustrate the failing Travis builds that have since stopped happening).

[lukeswitz]

Closing/reopening on a fresh branch now that the Travis builds are working to remove unnecessary commits.