Add information about Collect backups

On Android 6+, shared preferences are currently backed up to Google Drive. Currently, no files are backed up.

Once we move to storing files in getExternalFilesDir, those will be backed up as well, up to 25mb.

Start by locating the Collect documentation section where backup and storage behavior is described. Document the Android 6+ shared-preferences backup behavior, the current lack of file backups, and the stated 25 MB limit for files stored in getExternalFilesDir; done means readers can understand what Collect backs up.

Written by the indexing model from the issue text.