Support RSA encrypt using private key and PKCS#1 paddding.

Some apps such as OpenVPN Connect for some reason generate
PKCS#1-padded RSA signatures using Cipher (initialized in
Cipher.ENCRYPT_MODE with private key!) instead of using Signature.

Unfortunately, RI supports this strange behavior and previous releases
of Android Keystore did the same. So, we have to continue supporting
this craziness.

Bug: 22083023
Change-Id: Ife3950d0d4ceb4e44e08014635312c1252878b69