On-device signing: Make sure Keystore key has the correct boot level.

While Keystore itself enforces that these keys can only be created/used
during a certain boot level, we need to verify ourselves that the key
actually *has* the correct boot level.

Bug: 167516462
Test: atest
Change-Id: I096d63323e0faf7a97ec5f571d2fdcee75d47c6e
1 file changed