Add SettingsRepo, refactoring & setting for battery limit

[miguelguzmanr]

Things to note:

• Still testing these changes. My sincerest apologies if anything broke.
• Could not decide on a name or correct folder for the SettingsRepo class. Please change.
• Could not decide on a proper name for functions that use "cat"/"echo" commands in the ShellExecutor class to read and write settings. Not sure if they're even useful at all, either, at least currently not very much. Please change.

[miguelguzmanr]

I suggest we put these changes on hold while we review other pull requests @langerhans

[miguelguzmanr]

Introduces configurable charging separation. Requires thorough testing, as this may introduce additional battery drain due to having to monitor the battery level.

[miguelguzmanr]

This approach only works if the app is running in the background. I think this might need to be implemented as a foreground service or a background task.

[langerhans]

Even with the fix from #23 this does not work in background? Could we just watch battery state with Intent.ACTION_BATTERY_CHANGED?

[miguelguzmanr]

Introduces a service to monitor the battery level. Just trying out a different approach. Sadly, the service forcefully ends when the app is terminated in any way.

Requires #23 to work in the background.

Fixed incorrectly reading restrictCharge setting as a system setting instead of as a text file. Discovered that charging separation briefly turns off when the screen is turned off. Not sure about the culprit. Added a trigger for screen off event to combat this behavior. Should work properly now.

Added an option to dump a log file for debug builds. Might be useful for troubleshooting issues.

[miguelguzmanr]

@langerhans please let me know your thoughts!

[miguelguzmanr]

@langerhans please let me know if this approach works better!

[langerhans]

Yup looks good to me, LGTM