FileHandlerActivity: add ActionBar back button in case of crashes

Sometimes, something isn't handled quite right and FileHandlerActivity
stays open instead of just disappearing. This provides an ActionBar back
button to give the user some place to go when this happens.
set titles of Share screen based on action: encrypt, sign, debug log
EncryptFileActivity: use Dialog to show error msgs, like other ops

The other operations have a showError() method for popping up an OK dialog
with an error message.  Yes, this is very Windows 3.1, but it works well
enough for now.
EncryptFileActivity: fix crash after selecting the key to encrypt to

Because of the way that the other Activity calls onActivityResult(), it is
not possible to launch an Activity in that method.  My guess is that
onActivityResult() is called in the thread of the returning Activity. To
fix this, we use a Messenger to send a message to the GPG thread to launch
the rest of the encryption process.
REBASE add SignFileActivity for signing incoming files with default key
DecryptActivity: add ActionBar back button in case of crashes

Sometimes, something isn't handled quite right and DecryptActivity
stays open instead of just disappearing. This provides an ActionBar back
button to give the user some place to go when this happens.
REBASE FileHandlerActivity: add ActionBar back button in case of crashes