[Android] Storage Access Framework

[mikeskydev]

Meta-issue to track requirements for the new storage mechanisms needed for Storage Access Framework (SAF).

We currently use MANAGE_EXTERNAL_STORAGE permission, but this is discouraged by Google for anything other than file manager apps.

Use cases:

• Initial population of user data into target folder
• Migration of existing user data to new target folder
• Saving a Sketch
• Loading a Sketch
• Loading Images/Models/Videos
• Saving preferences
• Taking Snapshot/Video

Interaction points:

• Sketchbook panel
• Save camera
• Local Media Panel
• Camera tool

Current plan is to use /sdcard/Documents/Open Brush. Writing is fine here, but SAF needed for read if files are altered externally by user, or app is reinstalled.

Bonus points: can use SAF to access other app's folders in the Documents folder if it's a known location!

User additionally has the option to choose a different folder, so we need to store the chosen path. However, we can 'direct' them to select the correct directory by opening straight to it in SAF.