Select*KeyFragment rewrites

- moved all specific Public/Secret logic from the Adapter class into
  inner subclasses in the Fragments
- more versatile status display ("revoked", "expired", "can certify"...)
- applied view holder pattern
- query logic, including subqueries, moved into provider classes

Closes #375

OpenKeychain/src/main/res/values/strings.xml

@@ -523,5 +523,7 @@
     <string name="label_revocation">Revocation Reason</string>
     <string name="label_verify_status">Verification Status</string>
     <string name="label_cert_type">Type</string>
+    <string name="can_certify">can certify</string>
+    <string name="can_certify_not">cannot certify</string>
 
 </resources>