[build] use optional polyfill on Android, and upgrade to NDK r19

<experimental/optional> is not available anymore in newer STL versions, e.g. according to libc++'s TS deprecation policy: https://libcxx.llvm.org/TS_deprecation.html

circle.yml

@@ -15,6 +15,7 @@ workflows:
       - android-arm-v7-template:
           name: android-gnustl-arm-v7
           stl: gnustl_shared
+          image: android-ndk-r17c:1d5db0eb34
       - android-release:
           filters:
             tags:
@@ -519,8 +520,11 @@ jobs:
       stl:
         type: string
         default: "c++_static"
+      image:
+        type: string
+        default: android-ndk-r19:8e91a7ebab
     docker:
-      - image: mbgl/android-ndk-r17c:dd161efed6
+      - image: mbgl/<< parameters.image >>
     resource_class: large
     working_directory: /src
     environment:
@@ -582,7 +586,7 @@ jobs:
 # ------------------------------------------------------------------------------
   android-release:
     docker:
-      - image: mbgl/android-ndk-r17c:dd161efed6
+      - image: mbgl/android-ndk-r19:8e91a7ebab
     resource_class: large
     working_directory: /src
     environment:

platform/android/config.cmake

@@ -139,6 +139,7 @@ target_include_directories(example-custom-layer
 )
 
 target_link_libraries(example-custom-layer
+    PRIVATE optional
     PRIVATE -llog
     PRIVATE -lGLESv2
 )

platform/android/gradle/dependencies.gradle

@@ -4,7 +4,7 @@ ext {
             minSdkVersion    : 14,
             targetSdkVersion : 27,
             compileSdkVersion: 27,
-            buildToolsVersion: '27.0.3'
+            buildToolsVersion: '28.0.3'
     ]
 
     versions = [

vendor/optional.cmake

@@ -1,9 +1,8 @@
 add_library(optional INTERFACE)
 
-# This polyfill is really only needed for
-# Windows. We need to be careful to not leak
-# it into any of our public interfaces.
-if(WIN32)
+# This polyfill is needed for Windows and Android since these standard libraries don't ship with
+# usable versions of <experimental/optional>
+if(WIN32 OR MBGL_PLATFORM STREQUAL "android")
     target_include_directories(optional SYSTEM INTERFACE
         ${CMAKE_SOURCE_DIR}/vendor/optional/include
     )