[Android] Use Android SDK instead of OpenSSL where possible

[EgorBo]

This PR redirects all RandomNumberGenerator, MD5, Sha1-512, HMAC and AES APIs to use Android SDK instead of OpenSSL.

[ghost]

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[jonpryor]

Overall Aside: you cannot rely on "normal" JNI Local Reference semantics, in which JNI Local References are "automatically freed." You can't rely on this for two reasons:

1. The "root stack frame" is not Java. The "auto release of JNI Local References" only happens when returning to Java. Here, you're (presumably) P/Invoking from C# into a C/C++ function which calls into Java. No "auto release" will occur.
2. Android only allows ~512 JNI local references to exist at a time. If you exceed this count, the process will assert & exit.

Consequently, every time you invoke a JNI method which returns a JNI local reference -- which is most of them! -- you must ensure that you call JNIEnv::DeleteLocalRef() on the value. This includes jclass and jstring values, but does not include jmethodID values.

[EgorBo]

@jonpryor thank you so much, amazing feedback!

[EgorBo]

@jonpryor could you please re-review?

I've added error handling here and there but in general, the managed code on top of it is responsible for input validation like buffers' lengths, non-negative or out-of-bounds indices, etc.