Add Online Mode with automated DEM download

[mkrupczak3]

Description

Currently, the user must download a DEM for their Area of Operations. The process is somewhat confusing and is only possible to do on a computer with mouse and keyboard.

The program should be modified to have a new default online mode, where a DEM is automatically downloaded when a drone image is loaded.

User Story

As a user, I want the program to obtain DEM for me automatically, so that I can tap to locate without thinking about it.

[mkrupczak3]

PR #117 by @rdkgit was recently merged, adding a working first implementation of this feature in OpenAthenaAndroid. It's based on the code from his sucessful implementation of the feature in OpenAthena for iOS

Currently working on this feature on the master branch. Need to clean up the multiple new UI's, add automatic downloading for the loaded drone image, and provide a way to select between an offline and online mode

[mkrupczak3]

Recently reverted this PR to implement some minor bug fixes in minor versions from v0.20.0 to v0.20.2.

New PR #123 will re-implement this feature again once merged

[mkrupczak3]

PR was merged recently with basic manual downloading capabilities. Need to re-work the finite state machine of MainActivity.java and add automatic DEM selection and/or download system for the user's loaded drone image