docs: Updating Direct Boot N preview docs for DP2

Small name changes were made to Direct Boot APIs and constants,
so the preview docs are being updated to reflect this. Doc changes
will get published when DP2 is released (and not before).
Also fixing small bug with context.

Bug: 27907930 27781930
Change-Id: I75d6bf4efaa9a8bece60b6e2d794af3d075fc353