[layertracegenerator] stream layer trace to file

Write the generated layer trace entry by entry
so we don't hold the entire unserialized layer trace
in memory.

Fixes: 272158947
Test: presubmit
Test: open streamed trace in winscope
Test: cat /proc/`pidof layertracegenerator`/status
old
VmHWM:	  959496 kB
new
VmHWM:	   21416 kB

Change-Id: I4bf0cd8fe98d1e2a44fb6b7c85c4e06e586a52ed
diff --git a/services/surfaceflinger/Tracing/RingBuffer.h b/services/surfaceflinger/Tracing/RingBuffer.h
index 7e38c55..b41c65b 100644
--- a/services/surfaceflinger/Tracing/RingBuffer.h
+++ b/services/surfaceflinger/Tracing/RingBuffer.h
@@ -24,6 +24,7 @@
 #include <utils/Timers.h>
 #include <utils/Trace.h>
 #include <chrono>
+#include <fstream>
 #include <queue>
 
 namespace android {
@@ -73,6 +74,19 @@
         return NO_ERROR;
     }
 
+    status_t appendToStream(FileProto& fileProto, std::ofstream& out) {
+        ATRACE_CALL();
+        writeToProto(fileProto);
+        std::string output;
+        if (!fileProto.SerializeToString(&output)) {
+            ALOGE("Could not serialize proto.");
+            return UNKNOWN_ERROR;
+        }
+
+        out << output;
+        return NO_ERROR;
+    }
+
     std::vector<std::string> emplace(std::string&& serializedProto) {
         std::vector<std::string> replacedEntries;
         size_t protoSize = static_cast<size_t>(serializedProto.size());