Prepare for BoringSSL update.

This change tweaks things as needed so that the code will compile
against both the BoringSSL that's currently in Android and a version
from upstream. The BORINGSSL_201509 define is temporary to allow the
switch to happen without breaking the build and a followup change will
remove it.

Change-Id: I3d09b5644661353723803bcbda937d34455849a5

keystore-engine/android_engine.cpp

@@ -220,6 +220,9 @@ const struct rsa_meth_st keystore_rsa_method = {
   RSA_FLAG_CACHE_PUBLIC | RSA_FLAG_OPAQUE | RSA_FLAG_EXT_PKEY,
 
   NULL /* keygen */,
+#if defined(BORINGSSL_201509)
+  NULL /* multi_prime_keygen */,
+#endif
   NULL /* supports_digest */,
 };