Generating StrongBox backed keys

•Add FLAG_STRONGBOX when the generator spec requires it.
•Throw StrongBoxUnavailableException when the request
fails due to HARDWARE_UNAVAILABLE.
•Add PackageManager.FEATURE_STRONGBOX_KEYSTORE

Test: KeyStore CTS tests under development on an emulator
Bug: 63931634
Change-Id: I42d32b22981e43e504d30e5657d21ac555c71ebe