break out signing key fingerprint calculation into its own method

This will be needed for checking an incoming repo URL with a fingerprint
included to see if that uri/fingerprint already exists in the database.

src/org/fdroid/fdroid/ManageRepo.java

@@ -158,22 +158,8 @@ public class ManageRepo extends ListActivity {
                 server_line.put("inuse", R.drawable.btn_check_off);
             }
             if (repo.pubkey != null) {
-                try {
+                String fingerprint = getRepoFingerprint(repo);
-                    MessageDigest digest = MessageDigest.getInstance("SHA-1");
+                server_line.put("fingerprint", fingerprint);
-                    digest.update(Hasher.unhex(repo.pubkey));
-                    byte[] fingerprint = digest.digest();
-                    Formatter formatter = new Formatter(new StringBuilder());
-                    formatter.format("%02X", fingerprint[0]);
-                    for (int i = 1; i < fingerprint.length; i++) {
-                        formatter.format(i % 5 == 0 ? " %02X" : ":%02X",
-                                fingerprint[i]);
-                    }
-                    server_line.put("fingerprint", formatter.toString());
-                    formatter.close();
-                } catch (Exception e) {
-                    Log.w("FDroid", "Unable to get certificate fingerprint.\n"
-                            + Log.getStackTraceString(e));
-                }
             }
             result.add(server_line);
         }
@@ -244,6 +230,27 @@ public class ManageRepo extends ListActivity {
         return null;
     }
 
+    protected String getRepoFingerprint(Repo repo) {
+        String ret = null;
+        try {
+            MessageDigest digest = MessageDigest.getInstance("SHA-1");
+            digest.update(Hasher.unhex(repo.pubkey));
+            byte[] fingerprint = digest.digest();
+            Formatter formatter = new Formatter(new StringBuilder());
+            formatter.format("%02X", fingerprint[0]);
+            for (int i = 1; i < fingerprint.length; i++) {
+                formatter.format(i % 5 == 0 ? " %02X" : ":%02X",
+                        fingerprint[i]);
+            }
+            ret = formatter.toString();
+            formatter.close();
+        } catch (Exception e) {
+            Log.w("FDroid", "Unable to get certificate fingerprint.\n"
+                    + Log.getStackTraceString(e));
+        }
+        return ret;
+    }
+
     @Override
     public boolean onOptionsItemSelected(MenuItem item) {
         switch (item.getItemId()) {