Keystore SPI: Add SecurityLevelEnum to KeyProperties

This patch adds the SecurityLevelEnum to KeyProperties. This enum can be
used by the public API surface to express levels of enforcements of key
properties. And to select a designated residence for a newly generated
or imported key.

The values UNKNOWN and UNKNOWN_SECURE are used to convey to older target
APIs API levels that have not been defined when they where published.

Test: None
Change-Id: I88681f21b8a8ea9a383d32ba99f3ab7d7c8909c3