Keystore 2.0: Shim around the basic functionality of Keystore 2.0

This patch adds a shim around the Keystore 2.0 AIDL spec. The new shim
is modularized like the AIDL spec into the base Keystore module
Keystore2, the security level specific interface KeystoreSecurityLevel,
and the operation specific interface KeystoreOperation.

Other system maintenance specific interfaces have yet to be added.

Bug: 159476414
Bug: 171305684
Test: None
Change-Id: I070f73739e4b37ce10568939ac666e40b14a52a8