First version of automatic contact discovery.

TODO:
- Configuration (much of it)
- Enabled by default?
- Which keys to import? Current state: All non-revoked and non-expired with matching userid
- Search for keys if already known? Current state: yes, may cause traffic (configuration: only when wifi?)
- Update interval: Currently Android handles it, might be good (causes automatic refresh on new contact and stuff like that) or bad (too many of refreshes)

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

@@ -518,5 +518,7 @@
     <string name="unknown_algorithm">unknown</string>
     <string name="can_sign_not">cannot sign</string>
     <string name="error_no_encrypt_subkey">No encryption subkey available!</string>
+    <string name="info_no_manual_account_creation">Do not create OpenKeychain-Accounts manually.
+        For more information, see Help.</string>
 
 </resources>