Add KeyPairGenerator.EC backed by Android KeyStore.

The Android KeyStore backed KeyPairGenerator can generate EC key
pairs, but it cannot be instantiated via the standard JCA approach of
KeyPairGenerator.getInstance("EC", "AndroidKeyStore"). Instead, the
user must invoke KeyPairGenerator.getInstance("RSA",
"AndroidKeyStore") and then tell it to generate an EC key pair.

This CL fixes this weirdness.

The fix requires the introduction of late resolution of key algorithm
and default key size. Prior to this CL, these parameters were resolved
prior to KeyPairGenerator initialization, inside KeyPairGeneratorSpec.
In this CL, these parameters are resolved during KeyPairGenerator
initialization. This is fine because KeyPairGeneratorSpec should be as
dumb as possible and all the logic should reside in KeyPairGenerator
and lower layers.

Bug: 19018089
Change-Id: I114502356e6c9691518cf05b6d9eb0920b4fe0b2