Import a subset of libbase, build an AAR.

It's still quite a lot for just CHECK, LOG, and
DISALLOW_COPY_AND_ASSIGN. The strings stuff is only needed because r26
is too old to have the stdlib string split/starts_with/etc. Maybe I'll
end up wanting more later and this is a worthwhile start?

base/.gitignore

@@ -0,0 +1 @@
+/build

base/build.gradle.kts

@@ -0,0 +1,32 @@
+plugins {
+    id("ndksamples.android.library")
+}
+
+android {
+    namespace = "com.android.ndk.samples.base"
+
+    defaultConfig {
+        externalNativeBuild {
+            cmake {
+                arguments.add("-DANDROID_WEAK_API_DEFS=ON")
+            }
+        }
+    }
+
+    externalNativeBuild {
+        cmake {
+            path = file("src/main/cpp/CMakeLists.txt")
+        }
+    }
+
+    buildFeatures {
+        prefab = true
+        prefabPublishing = true
+    }
+
+    prefab {
+        create("base") {
+            headers = "src/main/cpp/include"
+        }
+    }
+}

base/src/main/AndroidManifest.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android">
+
+</manifest>

base/src/main/cpp/CMakeLists.txt

@@ -0,0 +1,16 @@
+cmake_minimum_required(VERSION 3.22.1)
+project(base LANGUAGES CXX)
+
+add_compile_options(-Wall -Werror -Wextra)
+
+add_library(base
+    STATIC
+    logging.cpp
+    stringprintf.cpp
+    strings.cpp
+)
+
+target_include_directories(base
+    PUBLIC
+    include
+)

base/src/main/cpp/include/base/errno_restorer.h

@@ -0,0 +1,42 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <errno.h>
+
+#include "base/macros.h"
+
+namespace android {
+namespace base {
+
+class ErrnoRestorer {
+ public:
+  ErrnoRestorer() : saved_errno_(errno) {}
+
+  ~ErrnoRestorer() { errno = saved_errno_; }
+
+  // Allow this object to be used as part of && operation.
+  explicit operator bool() const { return true; }
+
+ private:
+  const int saved_errno_;
+
+  DISALLOW_COPY_AND_ASSIGN(ErrnoRestorer);
+};
+
+}  // namespace base
+}  // namespace android