Keystore SPI: Deprecate encryption flag.

The encryption-required flag is only available in already deprecated
API KeyPairGeneratorSpec and KeyStoreParameter will be ignored from
Android S. Keys are and have been encrypted by default for a long time
and if additional binding to the LSKF is desired it can be requested
by KeyGenParameterSpec.Builder#setUserAuthenticationRequired(boolean).

Test: None
Change-Id: I5bd4acb4bba276decd1930ae2e96a55f95627e10