Allow update_engine to write BCB.

update_engine can trigger a factory-reset when the update to an older
version or an incompatible version requires it.

Bug: 28700985
TEST=Updated a device with a factory-reset required and the BCB was
written.

(cherry picked from commit 15105ce7)

Change-Id: I7d2efc0e7f164d618cbb3fe190882e4fa8a89bac