Require CMake 3.10 and use CMAKE_CXX_STANDARD

Removes the need to add -std= flags manually. CMake 3.10 is available
in Ubuntu 18.04, which is the oldest LTS version we support.

Source/Android/app/build.gradle

@@ -60,6 +60,7 @@ android {
     externalNativeBuild {
         cmake {
             path "../../../CMakeLists.txt"
+            version "3.10.2"
         }
     }