This CL adds variants of the public key retrieval and signing
routine which use the WiFi Keystore HIDL for the backend.
The Android.mk has been modified to build a second variant of
the library to expose this HIDL backend. While here, add
guards to all headers.
Bug: 34603782
Test: Able to connect to wifi passpoint networks.
Change-Id: I444ef383e4d3fdabc10c3e44c1bae9747613c8cf
Create a pure virtual class "KeystoreBackend" which supplies the
crypto methods used by android_engine. Create a KestoreBackendBinder
class which implements the binder backend as a no-op change that
will allow future backends to be added.
Bug: 34603782
Test: Compiles
Change-Id: I16620aba569bd53290145b2b30242c4888106d0a