handle EXTRA_TEXT in a SEND Intent by writing to txt file then encrypting

This allows you to send arbitrary text to GPG for encryption and signing.

refs #1454
if EncryptFileActivity is started without a key/email, use the default key

The default key is the first secret key, and if that doesn't exist, the
first public key.  In the future, gpg should prompt the user for which key
to use.  This is really just placeholder functionality until there is a
proper key chooser.

refs #1633
SEND_MULTIPLE is not implemented yet so do not receive it

refs #1454 refs #1631
all IntentFilters should have category.DEFAULT, and safe ones .BROWSABLE

category.android.intent.category.DEFAULT means that the Activity can be
considered as a default for whatever matches the IntentFilter.

android.intent.category.BROWSABLE means that the Activity can be safely
invoked by clicking something in the browser.
PrivateFilesProvider: add debugging logging for the unsupported methods

This allows us to see what the apps that use this ContentProvider are
asking for.