Eliminate unnecessary work for empty LSKFs

Improve the performance of creating and unlocking the LSKF-based SP
protector when the LSKF is empty by using the minimum scrypt parameters,
and by skipping the Gatekeeper step when Weaver is unavailable.

This also has the "benefit" that it avoids depending on auth-bound keys
working correctly for the device to boot when the LSKF is empty.  That
is relevant when an OEM has not implemented auth-bound keys correctly.

Bug: 232452368
Bug: 251131631
Bug: 251147505
Bug: 251398909
Change-Id: Ic58580455a789d177c077d12efe5ab6c0ec2085b