Fix Android Keystore KeyPairGenerator for RSA PSS keys.

This fixes a bug where key pair generation fails for RSA signing keys
which are not authorized for PKCS#1 signature padding, such as keys
authorized only for the RSA PSS signature padding scheme.

The issue was that the KeyPairGenerator was failing when attempting to
sign the self-signed certificate (needed by Android Keystore) using
PKCS#1 padding for which such keys are not authorized. The solution is
to not attempt to sign these certificates and instead use a fake
signature.

Bug: 21809600
Change-Id: I4f04fcf78174937046d2534e0485c6940eae673f