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
