Create android-base/format.h that includes fmtlib headers

We're already including fmtlib statically in libbase, so let's also
expose its functionality directly.

Also create a small benchmark of the most common operations for
format() and StringPrintf(): int, float, and string arguments.  Note
that fmt::format is faster than StringPrintf() in all of these cases.

Test: build, run benchmark
Change-Id: I1e18f13673dd89ea936ea5c51418fad723495b08
diff --git a/base/Android.bp b/base/Android.bp
index 7a2e665..58b6fb5 100644
--- a/base/Android.bp
+++ b/base/Android.bp
@@ -182,3 +182,21 @@
     },
     test_suites: ["device-tests"],
 }
+
+cc_benchmark {
+    name: "libbase_benchmark",
+    defaults: ["libbase_cflags_defaults"],
+
+    srcs: ["format_benchmark.cpp"],
+    shared_libs: ["libbase"],
+
+    compile_multilib: "both",
+    multilib: {
+        lib32: {
+            suffix: "32",
+        },
+        lib64: {
+            suffix: "64",
+        },
+    },
+}