C++98 (stlport) support for core FlatBuffers and FlexBuffers.

* Added internal - limited - implementation of flatbuffers::unique_ptr
  for STLs that don't ship with std::unique_ptr.  In C++11 and beyond
  this is just an alias for std::unique_ptr.
* Aliased used type traits structs is_scalar is_floating_point is_unsigned
  into flatbuffers namespace so they can be replaced in C++98 implementations.
  Right now these point at stlport's TR1 implementations.
* Wrapped vector::data() in vector_data().
* Wrapped vector::emplace_back() in vector_emplace_back().
* Wrapper string::back() in string_back().
* Added variants of FlatBufferBuilder::CreateVector() and
  FlatBufferBuilder::CreateVectorOfStructs() that allow the use of plain
  function pointers.
  Generated code has also been modified to use plain functions to build objects
  rather than std::function() so all generated code will work in C++98
  applications.
* Added flexbuffers::Builder::Vector(), flexbuffers::Builder::TypedVector()
  and flexbuffers::Builder::Map() methods that allow the use of plain function
  pointers.
* Changed Parser to internally use plain function pointers when parsing table
  and vector delimiters.
* Added specializations of NumToString() for 64-bit types that aren't supported
  by stringstream in stlport.
* Overloaded numeric_limits for 64-bit types not supported by stlport.
* Replaced build_apk.sh (which was broken by deprecation of the
  "android" tool in the Android SDK) with build.gradle and the
  appropriate gradle wrapper to build an APK.
* Switched Android build to build against all STL variants.
* Updated travis configuration to build Android test and sample.

Tested:
* Verified all tests continue to work on Linux, OSX and Android.
* Verified Travis build is green.

Change-Id: I9e634363793f85b9f141d21454b10686020a2065

samples/android/build.gradle

@@ -0,0 +1,108 @@
+// Copyright (c) 2017 Google, Inc.
+//
+// This software is provided 'as-is', without any express or implied
+// warranty.  In no event will the authors be held liable for any damages
+// arising from the use of this software.
+// Permission is granted to anyone to use this software for any purpose,
+// including commercial applications, and to alter it and redistribute it
+// freely, subject to the following restrictions:
+// 1. The origin of this software must not be misrepresented; you must not
+// claim that you wrote the original software. If you use this software
+// in a product, an acknowledgment in the product documentation would be
+// appreciated but is not required.
+// 2. Altered source versions must be plainly marked as such, and must not be
+// misrepresented as being the original software.
+// 3. This notice may not be removed or altered from any source distribution.
+
+buildscript {
+  repositories {
+    jcenter()
+  }
+  dependencies {
+    classpath 'com.android.tools.build:gradle:2.3.0'
+  }
+}
+
+allprojects {
+  repositories {
+    jcenter()
+  }
+}
+
+apply plugin: 'com.android.application'
+
+android {
+  compileSdkVersion 25
+  buildToolsVersion '25.0.2'
+
+  sourceSets {
+    main {
+      manifest.srcFile 'AndroidManifest.xml'
+      res.srcDirs = ['res']
+    }
+  }
+
+  externalNativeBuild {
+    ndkBuild {
+      path "jni/Android.mk"
+    }
+  }
+
+  defaultConfig {
+    applicationId 'com.samples.FlatBufferSample'
+    // This is the platform API where NativeActivity was introduced.
+    minSdkVersion 9
+    targetSdkVersion 25
+    versionCode 1
+    versionName "1.0"
+
+    buildTypes {
+      release {
+        minifyEnabled false
+      }
+    }
+
+    externalNativeBuild {
+      ndkBuild {
+        targets "FlatBufferSample"
+        arguments "-j" + Runtime.getRuntime().availableProcessors()
+        abiFilters "armeabi", "armeabi-v7a", "arm64-v8a", "x86", "x86_64"
+      }
+    }
+  }
+
+  lintOptions {
+    abortOnError false
+  }
+
+  // Build with each STL variant.
+  productFlavors {
+    stlport {
+      applicationIdSuffix ".stlport"
+      versionNameSuffix "-stlport"
+      externalNativeBuild {
+        ndkBuild {
+          arguments "APP_STL=stlport_static"
+        }
+      }
+    }
+    gnustl {
+      applicationIdSuffix ".gnustl"
+      versionNameSuffix "-gnustl"
+      externalNativeBuild {
+        ndkBuild {
+          arguments "APP_STL=gnustl_static"
+        }
+      }
+    }
+    libcpp {
+      applicationIdSuffix ".libcpp"
+      versionNameSuffix "-libcpp"
+      externalNativeBuild {
+        ndkBuild {
+          arguments "APP_STL=c++_static"
+        }
+      }
+    }
+  }
+}