Use decryption keys on decryption tests in CipherTest

Previously the test was using the encryption key for a decryption test,
which is just incorrect, and will fail when we start disallowing decryption
with public keys in RSA with OAEP.

Also factor the decryption key out into a variable to match how the
encryption key is handled.

Test: cts -m CtsLibcoreTestCases -t libcore.javax.crypto.CipherTest
Change-Id: I852ac1b45e8ed891ff35e81902cec6284c037e2e
1 file changed