binder: X509 are serialized to vector<uint8_t>

Change it from a string to a vector<uint8_t>. For DER
format, it is not a print-able string.

Test: pass
Bug: 198833574
Change-Id: I6b4d0ebc2d7429e927f7ca5abccc3f6aedfe1e6a
diff --git a/libs/binder/RpcTransportRaw.cpp b/libs/binder/RpcTransportRaw.cpp
index 62c9530..827e518 100644
--- a/libs/binder/RpcTransportRaw.cpp
+++ b/libs/binder/RpcTransportRaw.cpp
@@ -111,7 +111,7 @@
     std::unique_ptr<RpcTransport> newTransport(android::base::unique_fd fd, FdTrigger*) const {
         return std::make_unique<RpcTransportRaw>(std::move(fd));
     }
-    std::string getCertificate(CertificateFormat) const override { return {}; }
+    std::vector<uint8_t> getCertificate(CertificateFormat) const override { return {}; }
 };
 
 } // namespace