Improve stats_event memory usage

We now truncate the buffer to the appropriate length when clients call
stats_event_build().

Benchmarking tests indicate that truncating the buffer to the
appropriate length increases the cost clients pay to write to the socket
by 2%. This is negligible enough that I decided to truncate the buffer
for both pushed and pulled atoms in order to simplify the API.

Test: m libstatssocket
Test: bit libstatssocket_benchmark:*
Bug: 144126231
Change-Id: I35dec748ff87c0821d0d06779a406997e6e64966
Merged-In: Ife976bb383ecff8de5064730692a95e2a3a82c9d
diff --git a/libstats/socket/Android.bp b/libstats/socket/Android.bp
index bd3d9ae..94c405d 100644
--- a/libstats/socket/Android.bp
+++ b/libstats/socket/Android.bp
@@ -47,3 +47,23 @@
     export_include_dirs: ["include"],
     host_supported: true,
 }
+
+cc_benchmark {
+    name: "libstatssocket_benchmark",
+    srcs: [
+        "benchmark/main.cpp",
+        "benchmark/stats_event_benchmark.cpp",
+    ],
+    cflags: [
+        "-Wall",
+        "-Werror",
+    ],
+    static_libs: [
+        "libstatssocket",
+    ],
+    shared_libs: [
+        "libcutils",
+        "liblog",
+        "libgtest_prod",
+    ],
+}