Replace test RSA-512 keys with RSA-2048 (#1192)

* Replace test RSA-512 keys with RSA-2048

RSA-512 was factored in 1999, before Android even existed. Test with a
more plausible key size. Since generating RSA keys is a little slow
(0.1s), I've made NativeCryptoTest reuse a single generated test key.

* Move all test data initialization to a @BeforeClass

See discussion in PR

Author: davidben