Upstream Android-specific things under |BORINGSSL_ANDROID_SYSTEM|

The Android system BoringSSL has a couple of changes:
  ∙ ChaCha20-Poly1305 is disabled because it's not an offical
    cipher suite.
  ∙ P-521 is offered in the ClientHello.

These changes were carried in the Android BoringSSL repo directly. This
change upstreams them when BORINGSSL_ANDROID_SYSTEM is defined.

Change-Id: If3e787c6694655b56e7701118aca661e97a5f26c
diff --git a/ssl/t1_lib.c b/ssl/t1_lib.c
index caa0cc4..f30e8eb 100644
--- a/ssl/t1_lib.c
+++ b/ssl/t1_lib.c
@@ -352,6 +352,9 @@
 static const uint16_t eccurves_default[] = {
     23, /* X9_62_prime256v1 */
     24, /* secp384r1 */
+#if defined(BORINGSSL_ANDROID_SYSTEM)
+    25, /* secp521r1 */
+#endif
 };
 
 int tls1_ec_curve_id2nid(uint16_t curve_id) {