[mono] Add basic OpenSSL support for Android

[EgorBo]

Temp solution to enable System.Security.Cryptography.Native for Android while we figure out how to build/consume it properly.

Steps to enable:

1. Download and unzip https://maven.google.com/com/android/ndk/thirdparty/openssl/1.1.1g-alpha-1/openssl-1.1.1g-alpha-1.aar
2. Set these env variables:

export AndroidOpenSslHeaders="/Users/egorbo/prj/openssl-1.1.1g-alpha-1.aar/prefab/modules/ssl/include"
export AndroidOpenSslCryptoLib="/Users/egorbo/prj/openssl-1.1.1g-alpha-1.aar/prefab/modules/crypto/libs/android.x86_64/libcrypto.so"
export AndroidOpenSslLib="/Users/egorbo/prj/openssl-1.1.1g-alpha-1.aar/prefab/modules/ssl/libs/android.x86_64/libssl.so"

(make sure you use the right ABI in the paths, e.g. android.x86_64 for -arch x64 or android.arm64-v8a for -arch arm64)

3. build repo normally, e.g.

./build.sh -os Android -subset Mono+Libs

I am preparing a PR to update the docs and will include these temp hints.

[ghost]

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