rename MESSAGE_OKAY to MESSAGE_OK

This mirrors the Android style, e.g. RESULT_OK, etc.
split out EncryptFileActivity's process into separate methods

This makes the process a lot clearer, and makes it easier to share the
Handler and Messenger once the Fragment/AsyncTask for running GnuPG.gpg2()
is in place.
run gpg2 operations in a custom AsyncTask+Dialog fragment

GnuPG.gpg2() always needs to be run in a thread, otherwise it'll block
PinEntryActivity from starting up, if its needed.

fixes #1560
add option to sign when encrypting using EncryptFileTo

refs #1438
ImportFileActivity: split out actions from Handler into methods

Just to make things clearer, especially as this class is ported to use
the Gpg2TaskFragment.
convert ImportFileActivity to use Gpg2TaskFragment and its AsyncTask

GnuPG commands need to be run in an AsyncTask, otherwise they block
the PinEntryActivity from being launched.

closes #1560