certify: revamp certifykeyactivity

show info on the key to be certified, along with a list of (selectable)
user ids. user ids are handed through to the signing service, but not
yet handled.

OpenPGP-Keychain/src/main/res/values/strings.xml

@@ -465,5 +465,6 @@
     <string name="label_secret_key">Secret Key</string>
     <string name="secret_key_yes">available</string>
     <string name="secret_key_no">unavailable</string>
+    <string name="section_uids_to_sign">User IDs to sign</string>
 
 </resources>