Support cross-UID access from AndroidKeyStore.

This is meant for exposing the pre-existing cross-UID access to keys
backed by the keystore service via higher-level JCA API. For example,
this lets system_server use Wi-Fi or VPN UID keys via JCA API.

To obtain a JCA AndroidKeyStore KeyStore for another UID, use the
hidden system API AndroidKeyStoreProvider.getKeyStoreForUid(uid).

To generate a key owned by another UID, invoke setUid(uid) on
KeyGenParameterSpec.Builder.

This CL does not change the security policy, such as which UID can
access/modify which UIDs' keys. The policy is that only certain system
UIDs are permitted to access keys of certain other system UIDs.

Bug: 23978113
Change-Id: Ie381530f41dc41c50d52f675fb9e68bc87c006de