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/xml/custom_pgp_contacts_structure.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<ContactsSource xmlns:android="http://schemas.android.com/apk/res/android">
+    <ContactsDataKind android:mimeType="vnd.android.cursor.item/vnd.org.sufficientlysecure.keychain.key"
+                      android:icon="@drawable/key_small"
+                      android:summaryColumn="data1"
+                      android:detailColumn="data2"/>
+</ContactsSource>