Fix deleting legacy key blobs

Since the keystore alias prefix USERSKEY was deprecated
Credentials.deleteUserKeyTypeForAlias tried to delete key the
remaining prefix first and if that failed tried to delete the
legacy prefix.
However, KeyStore.delete returns true if the key was deleted or
did not exist. So the first call to delete would return true
whether the key existed or not and the legacy alias would never be
deleted.

This patch introduces a new flavor of KeyStore.delete, that returns an
error code instead of a boolean. The caller can now distinguish
the nature of the failure. Credentials.deleteUserKeyTypeForAlias now
checks this return code and attempts to delete the legacy variant if
KEY_NOT_FOUND was returned.

Bug: 117818447
Change-Id: Ifae1f3dbb07d85d94f430ead2cdd3e39d22436a4