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.
rename MESSAGE_OKAY to MESSAGE_OK

This mirrors the Android style, e.g. RESULT_OK, etc.
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.
REBASE 'run gpg2 operations in a custom AsyncTask+Dialog fragment'
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