update permissions

[andrekir]

• allow user to deny permissions (warn instead of error);
• improve and better explain permission dialogs;
• comply with Google's requirements for background location access.

references:
https://developer.android.com/training/location/permissions#request-background-location
https://support.google.com/googleplay/android-developer/answer/9799150

[andrekir]

• added GPS sensor check;
• added 'location only' logic to make sure we have it before asking for background location;
• disabled WRITE_EXTERNAL_STORAGE (only used in debug).

[geeksville]

This pull request has been mentioned on Meshtastic. There might be relevant details there:

https://meshtastic.discourse.group/t/meshtastic-android-1-2-51-alpha/4503/1