Fix the build of gles3jni sample

Previously it was specifying ANDROID_PLATFORM for cmake instead of
ANDROID_PLATFORM_LEVEL.

gles3jni/app/build.gradle

@@ -19,7 +19,7 @@ android {
                     // pass command line arguments to cmake: our CMakeLists.txt needs it
                     // to load the right openGL lib (gles2 vs. gles3 )
                     // other ndk flags could also be configured there [cppFlags, cFlags...]
-                arguments "-DANDROID_PLATFORM=android-${platformVersion}",
+                arguments "-DANDROID_PLATFORM_LEVEL=${platformVersion}",
                           '-DANDROID_TOOLCHAIN=clang', '-DANDROID_STL=gnustl_static'
             }
         }