Added App Introduction Screens
Added three screens explaining what Meshtastic is, how to use the app and how to set up encryption. This is only shown on the first launch of the app - after completing the introduction the app returns to normal functionality.
Closes #333
Screenshots

Thank you for your reviews. I've addressed them all and committed the changes. In regards to the AndroidManifest file, it seems like my IDE reformatted the file so my changes aren't very clear. I've reverted the file back to the original, committed it, and then added my code to the latest commit. Hopefully, this resolves the issue.
I took care of some leftovers in the manifest. if you run into this again, a tip is to revert at the file level on your local copy. you can review the diffs in the IDE after. 👍