Track bouncycastle upgrade to 1.51

The new API in 1.51 will allow null return values.

Change-Id: I583690bf0740b2d0962f02edea3896e18626e548

tools/signapk/SignApk.java

@@ -621,8 +621,12 @@ class SignApk {
             this.type = new ASN1ObjectIdentifier(CMSObjectIdentifiers.data.getId());
         }
 
+        /**
+         * This should actually return byte[] or something similar, but nothing
+         * actually checks it currently.
+         */
         public Object getContent() {
-            throw new UnsupportedOperationException();
+            return this;
         }
 
         public ASN1ObjectIdentifier getContentType() {