Switch to RsaPublicKey.

We have been storing the CompOS public key as the raw modulus. But on
the CompOS side we actually need it as a DER-encodedn RSAPublicKey
(which encodes both modulus and exponent).

Bug: 190166662
Test: Use compos_key_cmd to generate a key, odsign accepts it.
Test: Use FakeCompOs to generate a key, compos_key_cmd accepts it.
Change-Id: Ifdc9dc236d2b41ba2ee6a1a04cd7fd6dc4b1a241
5 files changed