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/RpcServer.cpp b/libs/binder/RpcServer.cpp
index 7fa2f57..1d42fc3 100644
--- a/libs/binder/RpcServer.cpp
+++ b/libs/binder/RpcServer.cpp
@@ -141,7 +141,7 @@
     return ret;
 }
 
-std::string RpcServer::getCertificate(CertificateFormat format) {
+std::vector<uint8_t> RpcServer::getCertificate(CertificateFormat format) {
     std::lock_guard<std::mutex> _l(mLock);
     return mCtx->getCertificate(format);
 }