libcrypto_utils: Use accessors to manipulate RSA keys.

This removes a blocker to BoringSSL marking the RSA struct opaque.
Sadly, OpenSSL's RSA_set0_key setter is a little tedious to use
correctly with error-handling, but it works.

Along the way, replace the byte reversing code with BoringSSL's native
little-endian BIGNUM conversion functions.

Test: libcrypto_utils_test passes
Change-Id: I298a6360d76fc7793462cd55bd57bc673c4975b3