Faster HostConnection creation

- Implement queryGlString in a single guest->host call for most
  cases: preallocate large enough buffer instead of querying for
  its size.
- Don't copy std::string-s around when we only needs a cref

Change-Id: Iffc4e9e37a1f9010ab5dfd5a4c5528f783c41137
diff --git a/system/OpenglSystemCommon/HostConnection.h b/system/OpenglSystemCommon/HostConnection.h
index 6586647..c460343 100644
--- a/system/OpenglSystemCommon/HostConnection.h
+++ b/system/OpenglSystemCommon/HostConnection.h
@@ -90,7 +90,7 @@
     static gl_client_context_t  *s_getGLContext();
     static gl2_client_context_t *s_getGL2Context();
 
-    std::string queryGLExtensions(ExtendedRCEncoderContext *rcEnc);
+    const std::string& queryGLExtensions(ExtendedRCEncoderContext *rcEnc);
     // setProtocol initilizes GL communication protocol for checksums
     // should be called when m_rcEnc is created
     void setChecksumHelper(ExtendedRCEncoderContext *rcEnc);