perfetto: add string writer class for formatting strings

This class acts as a replacement for snprintf for most circumstances and
is far faster in performance sensitive contexts.

Change-Id: Idf0d81dd8446f9a8858b9aee15be6931d298454b
diff --git a/Android.bp b/Android.bp
index 4b1d646..a002229 100644
--- a/Android.bp
+++ b/Android.bp
@@ -2557,6 +2557,7 @@
     "src/base/string_utils.cc",
     "src/base/string_utils_unittest.cc",
     "src/base/string_view_unittest.cc",
+    "src/base/string_writer_unittest.cc",
     "src/base/task_runner_unittest.cc",
     "src/base/temp_file.cc",
     "src/base/temp_file_unittest.cc",