Fetch attachments while MessageCompose activity is running

Android allows other apps to access protected content of an app without requesting the
necessary permission when the app returns an Intent with FLAG_GRANT_READ_URI_PERMISSION.
This regularly happens as a result of ACTION_GET_CONTENT, i.e. what we use to pick content
to be attached to a message. Accessing that content only works while the receiving activity
is running. Afterwards accessing the content throws a SecurityException because of the
missing permission.
This commit changes K-9 Mail's behavior to copy the content to a temporary file in K-9's
cache directory while the activity is still running.

Fixes issue 4847, 5821

This also fixes bugs related to the fact that K-9 Mail didn't save a copy of attached content
in the message database.

Fixes issue 1187, 3330, 4930