KeyChain: Provide public & private keys

In order for the DevicePolicyManager to provide key generation
functionality, it has to return both the private and public keys
in form of a KeyPair.

Since the KeyChainService will perform the key generation on behalf
of the DevicePolicyManager (so that KeyChain will be the owner of
the generated keys outright), the DevicePolicyManager needs a way
to get both the private and public key representations from KeyChain.

A getKeyPair method is added that gets the private and public
key pair associated with a given alias from Keystore.
The getPrivateKey now delegates to the getKeyPair method and returns
only the private key.

Tested using existing CTS tests.

Bug: 63388672
Test: cts-tradefed run commandAndExit cts-dev -a armeabi-v7a -m CtsDevicePolicyManagerTestCases -t com.android.cts.devicepolicy.DeviceOwnerTest#testKeyManagement
Change-Id: I06b8511acd2049a0053ec8893de6de7429f7c92e