show the ActionBar search field when switching to "Find Keys" tab

The only thing to do in the Find Keys tab is search for keys on the
keyserver, so it makes to always show the search field when switching to
that tab.

refs #2808
show progress spinner when waiting to load the list views
remove unused Intent extras when creating tabs in MainActivity
convert SelectKeysActivity to be based on fragment ActionBar

Use a button in the ActionBar to trigger the action on the selected keys.
Currently, this is only "Encrypt File To...".

refs #1633
convert EncryptFileActivity to receive arrays of keyIds and emails

Now, to send recipients to encrypt the file to, add a long[] of keyIds of
the recipients Key IDs as Activity.EXTRA_UID, and optionally, a String[] of
email addresses for the final SEND Intent as Activity.EXTRA_EMAIL.  If no
recipient keyIds are specified, then launch SelectKeysActivity to let the
user choose some.

refs #1633