Use the android-21 SDK as documented in README-android.md

(cherry picked from commit ac28702504)

android-project/app/build.gradle

@@ -14,12 +14,12 @@ android {
         versionName "1.0"
         externalNativeBuild {
             ndkBuild {
-                arguments "APP_PLATFORM=android-19"
+                arguments "APP_PLATFORM=android-21"
                 // abiFilters 'armeabi-v7a', 'arm64-v8a', 'x86', 'x86_64'
                 abiFilters 'arm64-v8a'
             }
             cmake {
-                arguments "-DANDROID_PLATFORM=android-19", "-DANDROID_STL=c++_static"
+                arguments "-DANDROID_PLATFORM=android-21", "-DANDROID_STL=c++_static"
                 // abiFilters 'armeabi-v7a', 'arm64-v8a', 'x86', 'x86_64'
                 abiFilters 'arm64-v8a'
             }