ImportFileActivity: use MIME type if file extension is not recognized

If the data coming in has a MIME type that is supported, then it should be
imported regardless of the file extension.

refs #1455
add strings for the PGP MIME types, file extensions are not so simple...

for .sig, .skr, and .pkr, there is a direct mapping for file extension to
MIME type.  For .pgp, .gpg, and .asc, each file extension can map to
multiple file extensions... sigh... yes its true...

refs #1455 refs #1456
don't add the setup and install Activities to the history

once the user has gone thru them, they should forget they ever existed...

Perhaps we should also remove them from Recents, I'm not sure there.
route known key files to ImportFileActivity, all else goes to FileHandlerActivity

Unfortunately, this can't all be nice and neat since .asc, .gpg, and .pgp
files can all have multiple contents (keys, sigs, or encrypted data). Also,
since these Activities just serve as data routers, keep them from being
added to the history and recents.

refs #1455 refs #1456
add two example files for detached signatures: .sig for binary, .asc for ASCII
REBASE route known key files to ImportFileActivity, all else goes to FileHandlerActivity
make GnuPG.gpg2() return the POSIX exit value for testing success/failure

This lets us do quick and dirty things like "gpg2 --verify"
add basic VerifyActivity to check .sig files

This currently just works with file:// Uris, and probably only files that
come in via the .sig file association in AndroidManifest.xml.  This is just
a rough version to get the ball rolling. Once GnuPGData can handle files
and streams directly (#1581) then this should be converted to always use
Streams, e.g. by using getContentResolver().openInputStream(uri)

refs #1569
verify .sig and .asc detached signatures, and prompt to view file if good

This verifies the .sig and .asc "--detach-sign" format of files, provided
the signed file is present as well.  If that's successful, then it'll
prompt the user to view the file or not.

refs #1569