Revert "IVGCVSW-6867 Update guides with new ndk version, r23."

This reverts commit bc463f7badcbefccedd0a4766c10255ae949ed1c.

Reason for revert: NDK upgrade caused unit test failures

Change-Id: I6d5782dc8bf513f91c9ca5d9bff564f92c021cef
diff --git a/BuildGuideAndroidNDK.md b/BuildGuideAndroidNDK.md
index 1784056..5068a8f 100644
--- a/BuildGuideAndroidNDK.md
+++ b/BuildGuideAndroidNDK.md
@@ -24,15 +24,17 @@
  mkdir -p $HOME/armnn-devenv/
  cd $HOME/armnn-devenv/
  # For Mac OS, change the NDK download link accordingly.
- https://dl.google.com/android/repository/android-ndk-r23-linux.zip
- unzip android-ndk-r23-linux.zip
- export NDK=$HOME/armnn-devenv/android-ndk-r23
+ wget https://dl.google.com/android/repository/android-ndk-r20b-linux-x86_64.zip
+ unzip android-ndk-r20b-linux-x86_64.zip
+ export NDK=$HOME/armnn-devenv/android-ndk-r20b
  export NDK_TOOLCHAIN_ROOT=$NDK/toolchains/llvm/prebuilt/linux-x86_64
  export PATH=$NDK_TOOLCHAIN_ROOT/bin/:$PATH
 
  # You may want to append the above export variables commands to your `~/.bashrc` (or `~/.bash_profile` in Mac OS).
  ```
 
+* With the android ndk-20b, you don't need to use the make_standalone_toolchain script to create a toolchain for a specific version of android. Android's current preference is for you to just specify the architecture and operating system while setting the compiler and just use the ndk directory.
+
 ## Build Google's Protobuf library (Optional)
 
 * Clone protobuf: 
@@ -120,9 +122,9 @@
 git checkout $(../armnn/scripts/get_compute_library.sh -p) 
 ```
 * the Arm Compute Library: 
-  (Requires SCons if not previously installed: `sudo apt install scons`. scons 2.3 or above is required to build the library.)
+  (Requires SCons if not previously installed: `sudo apt install scons`)
 ```bash
-scons toolchain_prefix=llvm- compiler_prefix=aarch64-linux-android$ANDROID_API- arch=arm64-v8a neon=1 opencl=1 embed_kernels=1 extra_cxx_flags="-fPIC" \
+scons arch=arm64-v8a neon=1 opencl=1 embed_kernels=1 extra_cxx_flags="-fPIC" \
  benchmark_tests=0 validation_tests=0 os=android -j16
 ```