RebootEscrowKey: use bytes for equivalence

Since RebootEscrowKey started wrapping SecretKey, it is necessary to check
equivalence via a method that does not do object equivalence. Use the
underlying byte array in the tests. equals() could be implemented, but then
hashCode() would need to be implemented which would be based on the private
key.

Test: atest RebootEscrowDataTest
Change-Id: Ia5b0bb822dcd3e59b5898f390fa8670db0b8d4c9