Introduce KEYSTORE_FLAG_CRITICAL_TO_DEVICE_ENCRYPTION

This flag is used by system server to mark keys used during the
synthetic password auth flow. keys marked with this flag will not
be super encrypted because super encryption requires knowledge of
the synthetic password, causing a chicken-and-egg problem.

Bug: 35849499
Bug: 34600579
Test: cts-tradefed run cts-dev -m CtsDevicePolicyManagerTestCases -t com.android.cts.devicepolicy.MixedProfileOwnerTest#testResetPasswordWithToken

Change-Id: I474822f2e026f24ce6f6de1aa58b5012922f7b13