More Javadocs for AndroidKeyStore public classes.

This adds more detailed class-level Javadocs (incl. examples) for the
following public API of Android KeyStore facility:
* KeyPairGeneratorSpec,
* KeyGeneratorSpec,
* KeyStoreParameter,
* KeyStoreKeySpec.

This also clarifies what encryption at rest means.

Bug: 18088752
Change-Id: I9951a528c34dea322534763b596902a2b6ac64f9