Added build for Android

[TGRCdev]

This commit adds the ability to build for Android platforms. For Android builds, a few new Scons options have been added:

• android_arch (Valid options: armv7, arm64v8, x86, x86_64)
  • The target Android architecture to build for
• ndk_platform (Valid range: 18-29)
  • The Android API level to target. Defaults to 18 for 32-bit platforms, 21 for 64-bit platforms
• ANDROID_NDK_ROOT
  • The path to an NDK installation on your computer. By default, uses the system's environment variables to find this.

This commit also updates godot-cpp to master commit 123d9f0, which supports Android builds.

This commit was built on Windows 10 64-bit and Lubuntu Desktop 19.04. Tested on a Samsung Galaxy S10E. Closes #31

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~~NOTE: While this commit technically does add Android support, the Godot C++ bindings themselves do not support building for Android yet. I am currently working with other developers to bring Android compilation for godot-cpp, and when it is officially added, it will work fine with this commit.~~